Erratic Idle when warm

[Marcos Beltran]

Yup. You can accomplish more with a scan tool in fifteen minutes, than with a multimeter in four hours. Maybe eight hours.

Trying to fix a computer-controlled engine/vehicle without a scan tool is two steps short of hopeless. But some folks do get lucky.


Suppose the fuel tank vent is nearly plugged, somehow. The vent tube got disconnected, and packed-full of mud. Or the tube got crimped. Or whatever.

You run the truck, your pump moves fuel out of the tank...but since the vent is almost totally plugged, no air gets in. Eventually, you drive long enough to create a vacuum in the tank. Now the fuel pump has to fight the vacuum in order to push fuel out. Fuel pressure goes too low, truck has driveability problems. You might even see on the scan tool that the short- and long-term fuel trims go rich-command as the tank builds vacuum.

Shut it off for a few hours, maybe you have enough leakage in the vent--or at the fuel filler cap--that the vacuum relieves, and the fuel pressure returns to normal.

Likely? No. But possible. The fuel tank vents via the charcoal canister, and long tubes.

Or--yeah--a failing pump that doesn't work right when it gets warmed-up. Thus my suggestion to verify fuel pressure in order to rule-out some weird stuff.

You could drive the thing until it acts-up, then remove the fuel fill cap. If the truck immediately runs better...

Ive been looking for a fairly decent priced OBD1 scanner but i cant find one. Do the OBD1 scanners give live data info the same way as an OBD2 scanner? I have an OBD2 scanner, i keep coming across adapter cables to adapt the OBD2 to a OBD1 but all have bad reviews. Is that even possible? All im finding on the internet are code readers for OBD1. Gonna look into fuel pressure and blocked vent. I did find a cracked hose at canister that i fixed. Easiest thing to test is taking off the fuel cap when i start having issues, will try that after work.

 

[Erik the Awful]

It seems like it has a vacuum leak but there isnt any and i would think that the vacuum leak would be there once you turn on the truck not 30 minutes later. So i started to suspect that it seems to start having problems once the truck is fully warmed up.

When the engine is cold, your fuel mixture runs richer, which can hide vacuum leaks. Once it's warmed up, your fuel mixture is less rich, and a vacuum leak is more noticeable.

 

[Marcos Beltran]

Check this out. Obviously this is not normal. I punch the accelerator real fast and the rpms seem to take forever to come down. Bad TPS? Ive replaced it already, maybe a crappy part. I replaced it a year ago and worked perfect. Started having same issues a few weeks ago and replaced the TPS again and ran fine for about a week then new issues started to arise but still resembled symptoms of a faulty TPS. You think these cheap a** parts are to blame. Maybe once the part gets heated up it starts to fail and once it cools down it works again. Now it seems to have issues once you start the truck and maybe the part is about give out completely. Im starting to think this new part aint worth a damn, its from Oreillys with lifetime warranty. Aint no point in having the warranty if the part doesnt even last a few days.

 

[Marcos Beltran]

When the engine is cold, your fuel mixture runs richer, which can hide vacuum leaks. Once it's warmed up, your fuel mixture is less rich, and a vacuum leak is more noticeable.

I have a homemade smoke machine. Gonna hook it up tomorrow and do a thorough search for one. Just did a visual search so far.

 

[Marcos Beltran]

So tested the fuel pressure and thats not the problem. Its showing 17psi when starting and 17psi when its warmed up and having the idle/ power issues. Never fluctuates, a constant 17 psi. Hooked my smoke machine up and found 2 vacuum leaks. Fixed those and still have the problem. So it was idling erratic again and unhooked the vacuum line to the map sensor and idle went up to 1000rpm, then plugged the line with my thumb and idle smoothes out to about 550rpm. This would typically lead you to think that the map sensor is bad right, but i tested another map sensor and same issue. Granted the other map sensor was used and it did come off of a parts truck. But both having the same problem? What else could cause this cause the vacuum line that goes to map sensor, the other end goes to the back of the tbi. Any ideas anyone?

 

[PlayingWithTBI]

Its showing 17psi when starting and 17psi when its warmed up and having the idle/ power issues. Never fluctuates, a constant 17 psi.

Huh? The "Stock" TBI S/B 9-13 (acording to GM) with 13-14 optimum. Are you running some go fast parts with an aftermarket pressure spring?

 

[Marcos Beltran]

Huh? The "Stock" TBI S/B 9-13 (acording to GM) with 13-14 optimum. Are you running some go fast parts with an aftermarket pressure spring?

Nope, all stock v6, 4.3 tbi.

 

[Schurkey]

Ever tested your fuel pressure gauge for accuracy?

Yes, a scan tool for OBD-1 provides similar data to a scan tool for OBD-2. What's available on the data stream, and what can be accomplished with bi-directional control of the scan tool over the vehicle computer depends on both the scan tool software, and the vehicle computer programming. In general, newer computers allow the scan tool more access and provide more data.

 

[Marcos Beltran]

Found the problem. It was a bad O2 sensor. This was kicking my a**, truck was down for over a month. Engine light never came on. Could not get any codes and tried hooking up to a buddy's scanner but his scanner could not connect or didnt have the right software which made this repair very difficult. Anyways, live and learn i guess.

 

[1997]

nice work and posting solution.