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1 GEN Trooper V6 FAST IDLE

852 Views 28 Replies 5 Participants Last post by  Ed Mc.
My swapped 3.4L V6 is idling about 1200-1800 RPM after its been running for about 5 min. The throttle body and intake are original from 2.8L but problem recently started. I have replaced the IAC but no effect. I do not see any open vacuum ports or hoses breached, and the throttle valve fully closes and is not sticking at idle, and timing checked.

I do not think there are any air leaks around TBI gasket and injector spray pattern normal. I am getting normal engine temps on gauges (but I suspect there is a 2nd temp sensor for ECM) and no check engine light.

Any suggestions appreciated
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Thanks yet again.
I don’t think the set screw would do much as fast idle is a recent issue long after engine swap. I did the TBI calibration above without much affect. Again, engine idles about right 700 ish rpm but a bit rough, but still climbs to about 1800 rpm after engine warms up. Unplugging IAC brings it down to about 1000 -1200 RPM. It also indicating a fault code 32 which relates to EGR which I don’t think would cause idle issue, and no code for faulty temp sensor which would have perhaps explained the higher rpm once engine is warm.

I replaced TPS years ago when trouble shooting rough idle after swap but will check ref 5v is getting to it.

I will look up suggested Vassel post And give it a try.

any other suggestions welcomed.
I remember trying OBD 1 readers and scanners years back and most only gave DTCs and not graphic or live data needed to really trouble shoot. …I will revisit available scanners. thanks

engine idle RPM continues to increase as engine warms up but cannot figure out why ECM is commanding it.

today, i again looked for vacuum leaks and replaced the MAP sensor, as well as spray ether around the intake in effort to find an air leak, but still no culprit to high idle. My next step is to check temp sensor resistance which could explain the higher RPM as temp increases, but doubt even a bad sensor would cause ECM to command such a high RPM. After that I am out of suspects and likely drop it off at a shop but it’s hard finding a shop experienced in older engines now a days.

Suggestions always welcomed.
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Thanks for advice all. The idle still increases to 1800-2100 RPM with warm engine, and believed it likely the temp sensor on manifold and even though resistance reads about right, I replaced it with no affect, as well as new IAC (and completed reset procedure without improvement).

because the idle is so wide, from 600 ish to 2000, I don’t think adjusting idle screw will do much and still thinking the ECM is commanding it high for some reason, but with new IAC and temp sensor. I also don’t think it’s a vacuum leak due to wide range of idle rpm and no increased rpm when spraying starting fluid around lines and gasket. I also swapped MAP sensor with no effect.

my only other thought may be leaky injectors, but Im not sure such a leak would give consistent symptoms.

Any other suggestions appreciated.
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Well, I have tried most everything I know and idle returns to about 1800 RPM after a short time, yet idles normal after start up. Also, if idling high and and restart the engine it returns to norm initially.

Here's what I have done so far:
- now IAC and calibrated it several times
-new temperature sensor.... I really thought this was issue
  • checked & cleaned PCV valve
  • check all vac lines by pinching each shut and spayed starting fluid around TBI gasket w/o affect
-tightened TBI bolts, checked for leaking injectors, cleaned valves and barrel
- swapped MAP sensor

Only thing left is to check or swap TPS. Can anyone tell me if the TPS calibration is rotating TPS to get about .5V @ closed position and 4.5'ish @ full open on the stock GM engine 2.8L

Interesting, the RPM does not change if I unplug IAC, but I can hear it closing when performing calibration.

After TPS I will likely try adjusting idle screw, but not sure it will help as I need to reduce idle by about 800-1200 RPM, and concerned that the normal idle I now get after starting will not sustain.

Any more thoughts appreciated.
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I pinched all vac lines off at the TBI early on. Cleaned out the PCV and it seems functioning as normal. If I cycle the key off/n when experiencing high idle it seems to reset it back closer to 700-900 rpm, so I think it an ECM issue rather than a vac issue at this point. Something is causing ECM to command higher rpm at some point but reset with key cycled…. Will have to likely get a OBD1 ALDL scanner and watch what happens. I have no more ideas.
will unplug ECS next drive but doubt its the problem as the original tested about right for resistance vs temp. WIll also try unplugging it while experiencing high idle
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