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Discussion Starter · #1 ·
Before I dig in a replace the IMG (intake manifold gasket) I wanted to reach out and see if anyone had a similar problem. On the first start of the day it will start and idle normal 800-1100 RPM. When I drive it any distance it will idle anywhere from 2000-3000 RPM. I have replaced most of the vacuum system to include the IAC valve/Throttle position sensor/EGR/PCV/MAP/Electronic Evap regulator valve. I am convinced this is the intake manifold gasket as these troopers have a history of failure.

However, when it is idling 2000-3000 RPM, if I take the MAP sensor electrical plug off the idle goes to normal but it starts to really smell like unburnt gas and black stuff comes out of the muffler. Any help is appreciated.
 

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Have you cleaned or replaced the MAF sensor? If that's bad or really dirty it could conceivably cause issues with idling, though you would probably also have some other problems. I've had good success with a good spray down with MAF sensor cleaner. Other than that you've already touched everything I would have thought to touch and then some. Good luck man.
 

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Have you taken a look at the ECS (Engine Coolant Sensor), the one screwed-into the top center of the intake manifold, forward of the TBI? They are notorious for going bad and causing problems. If the engine runs better with the sensor disconnected, it's bad. Here's a trusted part at Rockauto, a Beck Arnley, under $16 plus shipping:


On the GM TBI system, any time you change something like a sensor, exhaust backpressure, etc etc you have to re-calibrate the system. Especially if you've replaced the TPS. It's supposed to run in a specified voltage range and the engine will be very cranky if not adjusted properly.

Here are some directions on how to adjust the TBI's minimum (hard throttle stop) engine rpm, and this will also be a good check of how the IAC is working. An erratic IAC can cause the problems you're having, but if minimum idle speed and TPS aren't set right, the IAC can act up as well. BTW the reaction when you unplugged the MAP isn't surprising, since it's a critical sensor and without that input, the ECM has a hard time knowing what to do. Basically it'll be in "limp" mode, running a default "map" of parameters that, while they may keep the engine running, may not run it well.

Hat Tip to Jay Vessels, the author of this article, Long, Long, Ago and so very Far, Far, Away!

Adjust your TPS and I.A.C. on your TBI / MPFI

Authored by: Jay Vessels

Tools needed:

Torx bits or drivers (T-10, T-15, maybe more depending on the application)

Voltmeter (digital is best, but a really accurate analog will work)

Tachometer (the one in the vehicle will work fine if equipped)

Wrenches and an awl (various sizes, only if the idle speed hasn’t ever been set)

Theory of Operation – (lengthy)

A common myth about fuel injected vehicles is that the idle speed is fixed and cannot be adjusted. This isn’t quite true; there is a setting. It's called minimum air, which is adjustable on TBI and MPFI vehicles. This setting sets the lowest-possible idle speed for the vehicle. The ECM uses the IAC (idle air controller) to raise the idle speed from this adjustment. So, while the exact idle speed isn’t really adjustable, the minimum idle speed is.

Why adjust the idle speed? Isn’t the ECM supposed to do that? Yes it does and it does do a good job, but has to have a starting point. That starting point is called minimum air, or the smallest amount of air allowed to enter the engine with the throttle closed. The ECM can only add air to that minimum setting. If that setting is too high, the ECM can’t slow the engine down to an acceptable idle. If the setting is too low, the ECM may not be able to keep the engine running under certain conditions.

Another reason to adjust minimum air is if there has been some repairs to the fuel system. If the throttle body has been removed (i.e. rebuilt or cleaned) or the TPS (throttle position sensor) has been replaced or otherwise disturbed (i.e. loosened the mounting screws unintentionally -- it happens) then minimum air should be adjusted. Any changes that could affect idle speed or idle quality, like performance upgrades or replacing leaking vacuum lines, should be followed by setting minimum air.

This adjustment, once learned, only takes a few minutes. It rarely has to be adjusted, but it takes so little time to check (and adjust, if needed) that there’s no reason not to do so.

Checking & Adjustment Instructions

To establish minimum air, the idle speed must be set first. The idle speed screw is sealed with a cap from the factory. This should be removed by removing the throttle body and using an awl to pry the plug off. If this seems scary, have it done. It’s not difficult but it’s not worth risking damage to the throttle body or human flesh to remove the plug. Once the plug has been removed, reinstall the throttle body.

Assuming the idle speed screw is accessible and the throttle body is installed, jumper pins A&B on the ALDL (Assembly Line Data Link) connector under the dash. Pins A&B are on the upper-right-hand side. These are the same two pins to jumper to read codes from the ECM. Now turn the key on (the Check Engine light should be lit) and leave the key on for at least 30 seconds. The computer will extend the IAC plunger all the way out to allow adjustment of the idle speed.

(Note that on a Trooper, the ALDL plug is in the center console, under the pullout tray.)

After the 30 second wait, unplug the IAC (square 4-pin connector on the throttle body) WHILE THE KEY IS STILL ON. This prevents the ECM from adjusting the idle speed while you make your adjustments.

Block the drive wheels, set the emergency brake, and start the engine. Set the idle speed by adjusting the idle speed screw. The engine should be at operating temperature for this. The exact setting is on the emissions label on the radiator shroud, but in general, the idle speed should be about 500 RPM in Drive, 700 in Park / Neutral, or if you have a manual transmission, somewhere between 600-800 RPM. Remember that the truck is running during this adjustment, so stay clear of the fan, and make sure it can’t roll or otherwise be put into gear while this is done.

Once the minimum idle speed is set, turn the engine off, reconnect the IAC, and remove the jumper from the ALDL connector. The TPS minimum voltage must now be set. Turning the idle-speed screw may have moved the TPS idle voltage away from the specification, so it should be adjusted next.

Connect a voltmeter between pins A (usually dark blue) and B (usually black, or black/pink) of the TPS, and turn the key on. Don’t start the engine. Loosen the two torx screws holding the TPS in place, but don’t remove them. Rotate the TPS until the voltmeter reads between 0.45 and 0.55 volts, with 0.50 being ideal.

Tighten the mounting screws (carefully, they thread into soft aluminum) and re-check the voltage to make sure it’s still within range.


Attached below is a pic of the ESC; resistance vs temp curve for same; and the wires in the console. The ones with the creamy-color connector are for electronic spark control and these would be disconnected when you check/adjust engine base timing. The other set of wht/blu wires not connected, are for the Check Engine light feature and you'll be playing with these wires for the min idle speed and TPS adjustments. Those wires have the same function as jumpering the ALDL pins (as described in the procedure)

The last pic shows the plug for the idle stop screw. This is the plug you'll have to remove to be able to adjust idle speed. On mine, the plug was of very soft material. I just gouged it out with a small "tweaker" screwdriver. A scratch awl or similar tool should do as well.

HTH............ed

p.s. Welcome to the Planet!!



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Discussion Starter · #4 ·
Have you cleaned or replaced the MAF sensor? If that's bad or really dirty it could conceivably cause issues with idling, though you would probably also have some other problems. I've had good success with a good spray down with MAF sensor cleaner. Other than that you've already touched everything I would have thought to touch and then some. Good luck man.
to my knowledge, no MAF sensor just a MAP sensor and I did replace it. I will look at it again.
 

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Discussion Starter · #5 ·
Have you taken a look at the ECS (Engine Coolant Sensor), the one screwed-into the top center of the intake manifold, forward of the TBI? They are notorious for going bad and causing problems. If the engine runs better with the sensor disconnected, it's bad. Here's a trusted part at Rockauto, a Beck Arnley, under $16 plus shipping:


On the GM TBI system, any time you change something like a sensor, exhaust backpressure, etc etc you have to re-calibrate the system. Especially if you've replaced the TPS. It's supposed to run in a specified voltage range and the engine will be very cranky if not adjusted properly.

Here are some directions on how to adjust the TBI's minimum (hard throttle stop) engine rpm, and this will also be a good check of how the IAC is working. An erratic IAC can cause the problems you're having, but if minimum idle speed and TPS aren't set right, the IAC can act up as well. BTW the reaction when you unplugged the MAP isn't surprising, since it's a critical sensor and without that input, the ECM has a hard time knowing what to do. Basically it'll be in "limp" mode, running a default "map" of parameters that, while they may keep the engine running, may not run it well.

Hat Tip to Jay Vessels, the author of this article, Long, Long, Ago and so very Far, Far, Away!

Adjust your TPS and I.A.C. on your TBI / MPFI

Authored by: Jay Vessels

Tools needed:

Torx bits or drivers (T-10, T-15, maybe more depending on the application)

Voltmeter (digital is best, but a really accurate analog will work)

Tachometer (the one in the vehicle will work fine if equipped)

Wrenches and an awl (various sizes, only if the idle speed hasn’t ever been set)

Theory of Operation – (lengthy)

A common myth about fuel injected vehicles is that the idle speed is fixed and cannot be adjusted. This isn’t quite true; there is a setting. It's called minimum air, which is adjustable on TBI and MPFI vehicles. This setting sets the lowest-possible idle speed for the vehicle. The ECM uses the IAC (idle air controller) to raise the idle speed from this adjustment. So, while the exact idle speed isn’t really adjustable, the minimum idle speed is.

Why adjust the idle speed? Isn’t the ECM supposed to do that? Yes it does and it does do a good job, but has to have a starting point. That starting point is called minimum air, or the smallest amount of air allowed to enter the engine with the throttle closed. The ECM can only add air to that minimum setting. If that setting is too high, the ECM can’t slow the engine down to an acceptable idle. If the setting is too low, the ECM may not be able to keep the engine running under certain conditions.

Another reason to adjust minimum air is if there has been some repairs to the fuel system. If the throttle body has been removed (i.e. rebuilt or cleaned) or the TPS (throttle position sensor) has been replaced or otherwise disturbed (i.e. loosened the mounting screws unintentionally -- it happens) then minimum air should be adjusted. Any changes that could affect idle speed or idle quality, like performance upgrades or replacing leaking vacuum lines, should be followed by setting minimum air.

This adjustment, once learned, only takes a few minutes. It rarely has to be adjusted, but it takes so little time to check (and adjust, if needed) that there’s no reason not to do so.

Checking & Adjustment Instructions

To establish minimum air, the idle speed must be set first. The idle speed screw is sealed with a cap from the factory. This should be removed by removing the throttle body and using an awl to pry the plug off. If this seems scary, have it done. It’s not difficult but it’s not worth risking damage to the throttle body or human flesh to remove the plug. Once the plug has been removed, reinstall the throttle body.

Assuming the idle speed screw is accessible and the throttle body is installed, jumper pins A&B on the ALDL (Assembly Line Data Link) connector under the dash. Pins A&B are on the upper-right-hand side. These are the same two pins to jumper to read codes from the ECM. Now turn the key on (the Check Engine light should be lit) and leave the key on for at least 30 seconds. The computer will extend the IAC plunger all the way out to allow adjustment of the idle speed.

(Note that on a Trooper, the ALDL plug is in the center console, under the pullout tray.)

After the 30 second wait, unplug the IAC (square 4-pin connector on the throttle body) WHILE THE KEY IS STILL ON. This prevents the ECM from adjusting the idle speed while you make your adjustments.

Block the drive wheels, set the emergency brake, and start the engine. Set the idle speed by adjusting the idle speed screw. The engine should be at operating temperature for this. The exact setting is on the emissions label on the radiator shroud, but in general, the idle speed should be about 500 RPM in Drive, 700 in Park / Neutral, or if you have a manual transmission, somewhere between 600-800 RPM. Remember that the truck is running during this adjustment, so stay clear of the fan, and make sure it can’t roll or otherwise be put into gear while this is done.

Once the minimum idle speed is set, turn the engine off, reconnect the IAC, and remove the jumper from the ALDL connector. The TPS minimum voltage must now be set. Turning the idle-speed screw may have moved the TPS idle voltage away from the specification, so it should be adjusted next.

Connect a voltmeter between pins A (usually dark blue) and B (usually black, or black/pink) of the TPS, and turn the key on. Don’t start the engine. Loosen the two torx screws holding the TPS in place, but don’t remove them. Rotate the TPS until the voltmeter reads between 0.45 and 0.55 volts, with 0.50 being ideal.

Tighten the mounting screws (carefully, they thread into soft aluminum) and re-check the voltage to make sure it’s still within range.


Attached below is a pic of the ESC; resistance vs temp curve for same; and the wires in the console. The ones with the creamy-color connector are for electronic spark control and these would be disconnected when you check/adjust engine base timing. The other set of wht/blu wires not connected, are for the Check Engine light feature and you'll be playing with these wires for the min idle speed and TPS adjustments. Those wires have the same function as jumpering the ALDL pins (as described in the procedure)

The last pic shows the plug for the idle stop screw. This is the plug you'll have to remove to be able to adjust idle speed. On mine, the plug was of very soft material. I just gouged it out with a small "tweaker" screwdriver. A scratch awl or similar tool should do as well.

HTH............ed

p.s. Welcome to the Planet!!



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Wow man! Thanks! I know what I am doing the rest of the day. I thought these sensors were more plug and play I definitely didn't do any adjustments or break out my Fluke but I will. As soon as I figure it out ill let you know. Thanks again for all the information.
 

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NO mass airflow sensor on these engines (2.8 / 3.1 GM V-6s). The MAP sensor can go bad and cause drivability issues too. Almost all GM MAP sensors are the same from my experience.
Whatever Ed says, take as gospel. He KNOWS these engines. Dennis
 

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Thanks for the kind words, Dennis! Methinks you just might know a Thing or Three about ZuZu's, too!

Yeah, these OBD-1 systems can be a bit cranky if you mess with 'em. And it is indeed a MAP not MAF; the "Manifold Absolute Pressure" sensor is connected to a source of intake manifold vacuum, and connected electrically to the ECM, providing a signal that varies with vacuum pressure. And from what I've read, the diaphragm inside can be damaged by the pressure pulse of a backfire out of the Throttle Body.

There is a test you can do on the MAP, and it's one of those pressure (vacuum) vs electrical output things. This site explains it pretty well, and there are some other nice testing "nuggets" there if you dig around:


I'm 'sensing' more fun ahead (pun intended)!
😸
 

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Wow man! Thanks! I know what I am doing the rest of the day. I thought these sensors were more plug and play I definitely didn't do any adjustments or break out my Fluke but I will. As soon as I figure it out ill let you know. Thanks again for all the information.
Yeah, there ain't no Plug n' Play about OBD-1! But it is generally simpler and not horrible to troubleshoot. And no nasty On Board Spy Chips on board, that's a plus!
 

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Discussion Starter · #9 ·
Yeah, there ain't no Plug n' Play about OBD-1! But it is generally simpler and not horrible to troubleshoot. And no nasty On Board Spy Chips on board, that's a plus!
I am figuring that out now!

I have to say, I followed your directions to the T. She is running like a champ! The idle is about 750 rpm and she is responsive on the gas pedal with no high idle. The hardest part (for me) was getting the TPS to read .50 vdc, .51 was as close I could get it. I think it was a limitation of only having 2 hands. I drove her around for a bit without a single issue.

I cant thank you enough for stopping me from ripping the intake manifold gasket out.
 

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I am figuring that out now!

I have to say, I followed your directions to the T. She is running like a champ! The idle is about 750 rpm and she is responsive on the gas pedal with no high idle. The hardest part (for me) was getting the TPS to read .50 vdc, .51 was as close I could get it. I think it was a limitation of only having 2 hands. I drove her around for a bit without a single issue.

I cant thank you enough for stopping me from ripping the intake manifold gasket out.
There ya go, you're now a Boner-Fide OBD1 Mechanic!! Glad there weren't any strange problems beyond sensors and adjustments. Very rarely you might get a busted wire, or a short to ground, or lost ground. Those are the worst to find. But I've had my 3.4-swapped LS for almost 20 years and the only thing that's ever gone wrong on it (EFI-wise) is a bad temp sensor on the intake.

BTW, speaking of temp sensors, you may have noticed that your temp gauge doesn't come up much. Mine is lucky to get off the "C" peg in the Winter, in Summer months it'll run maybe 3/8 deflection at best. And I'm running the factory-specified 195F thermostat. Just the way it is, between the GM heat sender and the ZuZu gauges. The only time I ever saw a V6 temp gauge go over 1/2 was when my old 2.8 blew out heater hoses and the engine was steaming!! Tough little engine, I rolled down to the hardware store, patched 'er back together, filled with water and drove 'er home. Never blew a head gasket, either.

BTW .51V is Pretty Dang Good. Doesn't have to be perfect, long as it's within the range of what the ECM expects to see, it'll be happy. Just think how much you know now, compared to last week!! The Planet, what a site, eh!

And remember, just because you didn't have to pull the intake now, don't mean you won't have to do it someday! Keep those intake bolts tight and avoid coolant leaks into the oil. The bearings don't like that!

Enjoy!..........ed
 

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Discussion Starter · #11 ·
There ya go, you're now a Boner-Fide OBD1 Mechanic!! Glad there weren't any strange problems beyond sensors and adjustments. Very rarely you might get a busted wire, or a short to ground, or lost ground. Those are the worst to find. But I've had my 3.4-swapped LS for almost 20 years and the only thing that's ever gone wrong on it (EFI-wise) is a bad temp sensor on the intake.

BTW, speaking of temp sensors, you may have noticed that your temp gauge doesn't come up much. Mine is lucky to get off the "C" peg in the Winter, in Summer months it'll run maybe 3/8 deflection at best. And I'm running the factory-specified 195F thermostat. Just the way it is, between the GM heat sender and the ZuZu gauges. The only time I ever saw a V6 temp gauge go over 1/2 was when my old 2.8 blew out heater hoses and the engine was steaming!! Tough little engine, I rolled down to the hardware store, patched 'er back together, filled with water and drove 'er home. Never blew a head gasket, either.

BTW .51V is Pretty Dang Good. Doesn't have to be perfect, long as it's within the range of what the ECM expects to see, it'll be happy. Just think how much you know now, compared to last week!! The Planet, what a site, eh!

And remember, just because you didn't have to pull the intake now, don't mean you won't have to do it someday! Keep those intake bolts tight and avoid coolant leaks into the oil. The bearings don't like that!

Enjoy!..........ed
I am assuming this is a related issue or maybe something I have missed but now when I drive her down the road, it doesnt matter what gear I am in, if I push in the clutch the engine will spool down to normal range and kind of bounce up and down. attached is the video. any thoughts. I am going to re-check the minimum air at Idle and re-check the volts at the TPS as well but I thought this was interesting.
 

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Does it eventually settle down if you leave the clutch disengaged? They did have some strange stuff going on with emissions requirements, and the engine rpm slowly spooling up and down could have something to do with that. Or it might just be the ECM taking its time to re-learn its settings. Drive about 100 miles and see if it's still misbehaving.

There is a program you can use called WinALDL and it requires a patch cord to connect a laptop with the ALDL plug. You can get all sorts of useful info from the data stream. WinALDL - 160 baud ALDL reader

WinALDL is free, and you can either use an old laptop with a serial port and build your own inexpensive patch cord, or use a newer laptop with USB and buy a USB patch cord. Anyway, if it's still acting up later we can get into that. For now I'd just drive it.

One thing you could do that's easy, is to get the engine good n' hot, then let it idle and unplug the ECS sensor (on the intake manifold), and take a quick resistance reading with a good ohmmeter. Compare what you get to the temp vs resistance curve I posted, and it it's way out of spec, I'd replace the sensor. On both of my V6 rigs that same sensor has gone bad.
 

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Discussion Starter · #14 ·
I'll keep driving her for a bit to see if it gets better. I went ahead and replaced the ECS sensor on the intake manifold. Ill take some resistance readings soon to see what it looks like. After reading back through your initial comment I am not 100% sure I let the trooper get to temperature when making my adjustments. Its freezing in northern Nevada right now so I may just redo the procedure after I have let the Trooper warm up a bit. But I am not sure that would cause the fluctuation in engine speed or not.
 

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When it's really cold it might be hard to keep the ECM/engine in Closed Loop. The O2 sensor is mounted in the exhaust "wye" ahead of the cat, and it tends to cool off with prolonged idling. Since the ECM is relying on the O2 sensor input for Closed Loop operation, when it drops out and goes back to running in Open Loop on a pre-set "map", conditions ain't ideal for tuning.

One of our members " geoffinbc " wrote a good article about installing a heated O2 sensor. The internal heater helps the sensor get up to temp faster, and stay there. It'll run cleaner and idle better with the heated sensor keeping the ECM in Closed Loop.


Check out the rest of his site, he has a nice build with a stroked "3.2" GM V6 with "torquer" cam.

One old-fashioned way to keep the engine warmer in freezing/sub-zero weather is to block off part of the air intake, airflow to the radiator; in the old days we'd do that with a chunk of cardboard, but whatever you have on hand and suits your fancy. A few zip-ties and away you go. Or you can get real fancy with custom grille covers.

Here's a whole bunch of ideas for ya:


If you still have the OE-style air cleaner, make sure the heat stove is diverting warm air from the driver's side exhaust manifold up to the snorkel and into the TBI. This helps the engine warm up faster in really cold weather. There's a small thermostatic vacuum valve in the floor of the air cleaner that controls the vacuum motor on the heat "stove".
 

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Discussion Starter · #19 ·
I checked out the O2 sensor and it looked completely shot, probably the original. I went ahead and replaced it as well. The trooper idles completely fine, it will still rev when I take it out of gear after driving for a bit. It takes about 30-45 seconds of fluctuation before the engine stabilizes at around 1000-1800 rpm. another minute or two and it will idle down to 700-900. I think I will continue to drive her for another 100 miles before digging back in. In the meantime I have installed winALDL and am awaiting my usb-12pin cable. Thanks again Ed.
 

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That's taking way too long to settle down. Shouldn't be more than maybe 5-10 seconds or so.
 
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