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36 Posts
Discussion Starter · #1 ·
Finally I've managed to solve my high HC and erratic idle problems with both help from forum members and good old research into the operation and testing of automotive systems. The most important tools ended up being a vacuum gauge, fuel pressure tester, noid light, infrared temp tester and an inline spark tester.

Basically I was having the following problems:
Intermittant high idle at startup or when coasting. (Over 1500 RPM)
Idle surging when coasting. (Roughly between 750 and 1500 RPM which would settle down once stopped after about 10 - 15 seconds on average)
Rough idle.
Smelled like it was running rich.
Failing the low RPM part of the emissions test due to high HC.

The following details the inital test results, the work done and the results of the following test. I'm condensing text from previous posts to hopefully make it easier on folks looking for answers to their similar problem.

First test:
High Speed RPM = 2443
HC Reading 78 - Limit 180 Passed
CO Reading 0.53 - Limit 1.20 Passed
CO2 Reading 14.3
O2 Reading 1.22

Low Speed RPM = 0735
HC Reading 829 - Limit 180 FAIL
CO Reading 0.71 - Limit 1.20 Passed
CO2 Reading 13.1
O2 Reading 2.07

Work performed:
New NGK plugs
Plug wires
O2 sensor
Oil change
Remove and clean throttle body

Second test:
High RPM 2754
HC 108 Limit 180 - Pass
CO 0.73 Limit 1.20 - Pass
CO2 14.1
O2 0.78

Low RPM 737
HC 652 Limit 180 - FAIL
CO 0.74 Limit 1.20 - Pass
CO2 13.6
O2 1.29
No perceived change in engine performance
Low RPM HC dropped a bit

Work performed:
Remove and clean the entire intake (Very dirty)
Remove and send out the injectors for testing and cleaning (Dirty but otherwise ok except for one which was leaky)
Test injector wiring with NOID light (Pulse received on all injectors)
Test operation of EGR valve with vacuum pump and clean pintle. (Dirty pintle but otherwise ok)
Test vacuum transducer (Working ok)
Test VSV valve plus the solenoid valve next to it too (Working ok)
Checked and cleaned all vacuum lines (None clogged)
Cleaned the tube from the exhaust manifold to the EGR valve (Not clogged)
Checked PCV valve operation (Working ok)
New gaskets on intake and valve covers (Due to VC oil leak and high idle)
Replaced coolant temp sensor (Just because I had the intake off)
Replaced coolant manifold o-ring (Leaking)
New coolant
Oil change (Some coolant may have gotten into the cylinders when tearing things apart. Purely a precautionary measure)
Vacuum gauge tests (Pass)
Spark tests (Pass)

Third test:
High RPM 2338
HC 114 Limit 180 - Pass
CO 0.65 Limit 1.20 - Pass
CO2 14.1
O2 4.40

Low RPM 756
HC 457 Limit 180 - FAIL
CO 0.62 Limit 1.20 - Pass
CO2 13.8
O2 1.19
Engine most definitely much running smoother
Idle no longer high or surging
Added bonus: The lifter tick I've had for the last few years is now gone.
Low RPM HC dropped a bit more

Work performed:
Fuel presser test (Pass)
Infrared temp test of the catalytic converter (Fail - Outlet temp was lower than inlet temp)
Replace catalytic converter

Fourth test:
High RPM 2464
HC 21 Limit 180 - Pass
CO 0.01 Limit 1.20 - Pass
CO2 14.8
O2 0.03

Low RPM 752
HC 48 Limit 180 - Pass
CO 0.01 Limit 1.20 - Pass
CO2 14.9
O2 0.00
Engine runs even smoother and seems to have more power
Significant drops in HC, CO and O2

The primary cause of the engine idle surging problem was most likely due to a leaky intake manifold gasket. The high HC reading and rough idle was most likely a problem with multiple causes. A leaky intake gasket, along with a malfunctioning injector causing a rich condition which nailed the coffin lid on a cat that already had over 140,000 miles on it. Once the underlying problems were fixed the engine ran quite a bit better. Once the cat was replace it ran even smoother and passed the emissions test by a significant margin.

6,404 Posts
You pretty much wrote the exact story I went through in getting my LS to pass too.
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36 Posts
Discussion Starter · #5 ·
As far as costs go this is what I remember off the top of my head.
Plugs - ~ $2 ea.
Wires - don't remember but I think somewhere around $60 or so
Intake gaskets - ~$20
Spark plug tube seals - ~$22
Coolant temp. sensor - ~$25
Coolant manifold o-ring - ~$5
Fuel pressure guage kit - ~$40
Vacuum guage kit - ~$20
Universal catalytic converter installed - $150
Fuel injector cleaning and testing - $35ea.

Sorry for not remembering the exact prices on everything but this is in the ballpark. Hope it helps.

36 Posts
Godzillaface, many thanks for your efforts to provide a thorough report for your experience. About 11 years later, I'm walking in your shoes with a 1992 Trooper. Your report is a tremendously helpful guide.

56 Posts
Went through a bit of this but changed the cat right off.. got after market unit and welded it in myself,,saved me a bundle. But the benefits of forced repairs,,it runs much better now.. and it got the lowest numbers i have ever seen my trooper get at test.
Thanks for posting, these things are helpful to us all scratching our head's.
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