High Idle - Rich Exhaust

[Schurkey]

Use MAPP gas
Gas will maybe get pulled in through the leaks.
WD40 just plugs the leak long enough too lead down the wrong path.
WD40 is not a diagnostic tool.
It is a preseritive with great PR

I largely agree with you about WD-40 being mostly worthless. It displaces water and little else. I wouldn't even call it a preservative; since it's effects tend to be short-term.

There's no such thing as MAPP gas any more--hasn't been made since 2010 or so. What's sold in the yellow cylinders now is "MAP-Pro", a deliberately misleading name for a completely different fuel gas--essentially propane with some hot-sauce mixed in. Not that much hotter than plain propane. The FTC should make them change the name; but America doesn't have functional watchdog agencies. We have lapdog agencies that should receive hearty asp-kickings.

When I look for vacuum leaks, I use aerosol carb cleaner; and NEVER NEVER "brake" cleaner.

Temp runs at about 202°F on the scan. Had a 195°F in originally, it wasn't getting hot enough. switched to a 205°F

"It wasn't getting hot enough"...define "it". The engine? The heater duct temperature? The dash gauge?

A 205 thermostat has no business in a truck that works for a living, which I'd expect of a C3500.

I replaced the intake temp sensor, also checked for continuity of wire back to ECM and connections.

What does the scan tool show for coolant temp?

I only recently adjusted the idle set screw. I have it adjusted to 700 rpm in "Drive" for Minimum Idle Air Rate.

I would agree with the TPS. The manual says 0.6v, but also states less than 1.25v during diagnostics.
It is non-adjustable on the 7.4l However, before I replaced it, I slotted the last one with a dremel and was able to get it drop 20 points. But it didn't want to rise above 1.4v anymore either.

Find out what's wrong with the idle speed. You're using too much throttle; or the idle speed is too high. Thus the high TPS voltage at idle.

Does the spark advance work?

Curious about the "rebuilt" canister. Once upon a time, GM had considerable issues with canister purge valves; the canister would fill-up with fuel and the engine would be rich especially at idle. But that was several years before your truck was built.

 

[WV_Dave]

My 94 454 has a device called an idle speed control actuator and it's controlled by vacuum via the idle speed control actuator solenoid. Is this present on a 92?

 

[Jon_Bueno]

Just curious about the "rebuilt evap canister", did ya send it off to be rebuilt? Thanks.

No, I just cut it open and replaced the foam filters, refilled it with activated charcoal and resealed it with epoxy. The charcoal was for aquarium filters not quite as fine of material though but close enough for me.

 

[Jon_Bueno]

I largely agree with you about WD-40 being mostly worthless. It displaces water and little else. I wouldn't even call it a preservative; since it's effects tend to be short-term.

There's no such thing as MAPP gas any more--hasn't been made since 2010 or so. What's sold in the yellow cylinders now is "MAP-Pro", a deliberately misleading name for a completely different fuel gas--essentially propane with some hot-sauce mixed in. Not that much hotter than plain propane. The FTC should make them change the name; but America doesn't have functional watchdog agencies. We have lapdog agencies that should receive hearty asp-kickings.

When I look for vacuum leaks, I use aerosol carb cleaner; and NEVER NEVER "brake" cleaner.


"It wasn't getting hot enough"...define "it". The engine? The heater duct temperature? The dash gauge?

A 205 thermostat has no business in a truck that works for a living, which I'd expect of a C3500.


What does the scan tool show for coolant temp?



Find out what's wrong with the idle speed. You're using too much throttle; or the idle speed is too high. Thus the high TPS voltage at idle.


Curious about the "rebuilt" canister. Once upon a time, GM had considerable issues with canister purge valves; the canister would fill-up with fuel and the engine would be rich especially at idle. But that was several years before your truck was built.

Hey Schurkey -
Thanks for chiming in. You had a couple of great inquires. 1st thanks for keeping on my toes. I believe in being specific when I'm getting info also so, I'll clarify what I meant by "it". I meant engine coolant temperature. The coolant temperature would just hover around 190°F, I wasn't sure if the ECM was taking control over the system. I tried a couple different 195°F thermostats but it stayed at about 190°F. Someone suggested a thermostat with a higher temperature rating, as it is a "retired work truck" I gave it a shot. Currently the scan coolant temperature at idle is about 202°F.

2nd - I didn't really think to check the spark advance until you mentioned it. I'm not/wasn't getting a code 42-EST, and the truck seems to run smoothly, so I didn't think to check it. After checking the procedure from the manual, I believe my Ignition Control Module is faulty. If I'm not mistaken this could be a cause to my current conditions of "high idle" and "rich" exhaust?
I ordered a new ICM and will post my results after installation.

3rd - The "Rebuilt" Evap Canister is just refilled with fresh activated charcoal and new filters. Not sure if there is a "purge valve" on my "non-altitude" canister. It has a "purge line" also called a "canister purge". Air seems to flow freely from the fuel side and out the purge line though. I think that is how it should work, but I could be wrong.


Thank you for pointing me in what I hope to be the right direction. I assumed that when the distributor was replaced the module was replaced as well. But you know what they say assuming...

 

[thinger2]

I largely agree with you about WD-40 being mostly worthless. It displaces water and little else. I wouldn't even call it a preservative; since it's effects tend to be short-term.

There's no such thing as MAPP gas any more--hasn't been made since 2010 or so. What's sold in the yellow cylinders now is "MAP-Pro", a deliberately misleading name for a completely different fuel gas--essentially propane with some hot-sauce mixed in. Not that much hotter than plain propane. The FTC should make them change the name; but America doesn't have functional watchdog agencies. We have lapdog agencies that should receive hearty asp-kickings.

When I look for vacuum leaks, I use aerosol carb cleaner; and NEVER NEVER "brake" cleaner.


"It wasn't getting hot enough"...define "it". The engine? The heater duct temperature? The dash gauge?

A 205 thermostat has no business in a truck that works for a living, which I'd expect of a C3500.


What does the scan tool show for coolant temp?



Find out what's wrong with the idle speed. You're using too much throttle; or the idle speed is too high. Thus the high TPS voltage at idle.

Does the spark advance work?

Curious about the "rebuilt" canister. Once upon a time, GM had considerable issues with canister purge valves; the canister would fill-up with fuel and the engine would be rich especially at idle. But that was several years before your truck was built.

I have ten cans of actuall MAP gas left.
And about ten pounds of R12.
Im also in the bunker with 200 rolls of charmin toilet paper and 12 cases of good scotch
And no, you cant come over and squeeze any of it.

 

[Redduc1199]

My dad had a similar issue with his 5.7. He couldn’t get it to idle. The engine always acted like his foot was on the throttle. The TB gasket was torn at the intake manifold and it was sucking air causing the high idle.

 

[udidwht]

Get the 195 Delco thermostat in it. As for the TB idle follow the procedure in manual for setting the idle speed. One should never have to mess with the covered set screw. If you set it at 700rpm in drive and then throw her in park it will be higher for sure. The distributor ICM plug needs to be disconnected (single wire) then set the timing 4BTDC. Some of these do have a 'idle speed control actuator' does yours have one? When checking for vacuum leaks small shots of crab cleaner then listen carefully or watch a tach. Be sure the ground bracket at the T stat housing is clean including the bolts holding the T stat housing (2).

In the attached pic (my 1994 P-30) the 'idle speed control actuator' is lower left of the alternator. The sensor to the left of it is the 'Purge valve'. Distributor was replaced? With what distributor?

I eliminated the need for 'Charmin' squeezbly soft. Installed a bidet. Done.

 

[DerekTheGreat]

For what it might be worth, I once had a 1988 C3500 with a 454. It was a stickshift, so the idle strategy could be different on those. Idle RPM via MT2500 was 800 rpm. Long story short, I found somewhere that the idle on those is intentionally a bit higher than the small blocks to improve oiling to the cam and/or reduce stress on it. I went through all kinds of hoops to try and reduce the idle, but it always wanted to idle at 800, counts on the IAC supported that. Also, the TPS voltage is important. You need to keep that in range, it'll begin to command the IAC to open when above spec.

As for the O2 readings, is it still in closed loop or does it go open at idle? Have you tried installing a heated unit? I might do that with my truck. Below 1000 rpm (350, not 454), it pops out of closed loop. Cross counts are also down from what they should be. Fuel mileage is still good and I couldn't get it out the other day when I tried, so it's going to live there a little while longer.

 

[Schurkey]

According to the Emissions volume of the service manual set, my '88 K1500 5.7L is not supposed to drop out of closed loop at idle. I'm thinking that none of these engines are supposed to drop out of closed-loop at idle, but it's been a long time since I've read that part of the book, and I don't remember for sure.

With old O2 sensors, it would drop out of closed-loop. Absolutely seamless, the only reason I knew was because I had the scan tool connected. It'd pop back into closed-loop almost instantly upon application of throttle.

 

[PlayingWithTBI]

I'm thinking that none of these engines are supposed to drop out of closed-loop at idle, but it's been a long time since I've read that part of the book, and I don't remember for sure.

With old O2 sensors, it would drop out of closed-loop. Absolutely seamless, the only reason I knew was because I had the scan tool connected. It'd pop back into closed-loop almost instantly upon application of throttle.

Even with shorty headers, it MIGHT drop into OL until you give it some gas to help heat the one-wire O2. A heated O2 will ensure you stay in CL (as long as CTS is >150° IIRC).