Hi guys/girls...

I'm having a recurring problem with hard starts and high idling...

At first I thought the fuel pressure regulator, but it isn't. Fuel pressure holds fine, and actually goes up to 60psi when I shut the truck off???

So here are the symptoms:

High idle when first started from cold (~1100rpm), it will hold this rpm even when up to operating temps. When it does get to operating temps, at a stop, the engine bogs down then revs up to 1100rpm again. It's almost like it's fighting itself to get down to the right idle. When it's doing this, I can drive, but when I get to a speed where 4th gear engages, it keeps downshifting to third then back to fourth constantly, and the TC lock does not engage. I have to shift into 3rd gear in fear of screwing up the tranny (not sure if it will, but it doesn't hurt to be cautious).

Once in a while when it does the bog at a stop (or in park), the rpms will settle down to ~600 where it should be. When the rpms are fine, the truck drives like a dream. No constant shifting, and the TC locks in at cruising speeds.

This happens every morning, just inconsistent as to how long it lasts. Sometimes it lasts my whole ride home, sometimes it stops doing it a few blocks from where I start, but it does it all the time.

Second problem, which might be related to the first:

Sometimes it is hard to start. Only happens if the truck has been running a bit. Engine temps seem to not make a difference. It has done it when motor was still cold, warm, hot, you name it. When it doesn't start, I push the gas pedal down while cranking, and it fires up instantly. I only have to "blip" the gas to WOT, not really hold it there. Other times, I can start and shut off the truck no problems. Kind of like today when I'm trying to diagnose... Damn thing is running perfect LOL...

So yeah I'm at a loss here... I'm also wondering why the fuel pressure goes from 50 to 60 when truck is shut off???

INFO:

1997 4dr 4x4
fuel pump/filter changed in august (~3 months old) with delco parts
IAC cleaned last month
FULL tune up done last month (plugs, wires, cap, rotor)
Fuel pressure 60 key on, 60-65 starting, 50 idle, 60 key off.

Long post, but I want to be thorough.

Thanks for any suggestions!

 

Sounds like you have IAC problems. If cleaning it did not help, then the next step will likely be replacement of the IAC valve.

The TPS sensor is easy enough to test. Should output around 0.5V at closed throttle and 4.5V at wide open throttle.

You may also want to R&R the EGR system. A good diagnostic step is to make up a block off plate out of some sheet metal. Sandwich it between two new gaskets and bolt the EGR valve back on. See if the symptoms persist. This will cause a CEL after a bit so you shouldn't run with it this way for the long term.

For a back stories:
Hard to start when warm?
Stalling problem when cold...
It really helps to keep everything in one thread...

 

Sorry about the multiple threads... I thought I had the problem beat already...

Anyways, I will check the TPS and see about checking the IAC, maybe cleaning it again in the morning. I will post back my findings.

 

Ok, so I finally had time to do some more testing...

Turned it on from dead cold, idle is at 1K rpm. About 2 minutes of idling, I heard a drop in engine speed, and saw the tach moving slowly down as it should on a cold start. When it hit about 800rpm, it shot back up to 1K, and started doing that surging thing like it's fighting itself to get back down to normal rpms.

IAC functions normally. I had it out of throttle body and turned the key to on, and the pintle moves in and out freely. I know these are self resetting iac's so on turning ign off it extended far out to it's limits, then pulled back in to rest position. It's also still very clean from the last time I cleaned it.

TPS I may have an issue. It tested 0.56v closed, and 4.35v at WOT. No droppages when going from closed to open and vice versa, just smooth transitions.

Fuel pressure test done AGAIN (just in case lol) key on 60, holds for 30 minutes (that's how long I left it, didn't budge at all). Relieved pressure to 10psi, held that for 30 minutes.

So the only thing that's off is the TPS, but something doesn't make sense to me... With the high idle I get it to operating temp, shut it off for a minute of two. When I start it, I have to push the gas a bit to get it to fire, then the rpms settle down and everything is happily humming along. Took it for a drive when the rpms were right, and everything was nice, trans not jumping around, no issues.

Could a coolant temp sensor be a problem as well? How would one test this?

Thanks.

 

The IAC moving while out of the throttle body does not tell you if it is getting stuck while it is moving when in the throttle body or that it can be relied upon to accurately control itself during operating conditions.

Fuel pressure sounds perfectly fine.

The ECT sensor does not really play into idle speeds much and really shouldn't cause an issue with bouncing idle. You should be able to watch the ECT signal with a scan tool.

 

Had a chance to check it again Sunday. Took out the iac to see if any carbon build-up in there might be a problem. Didn't see any, but sprayed it down anyways. Started it up, still with the idle problem.

Drove around, running a few errands, the whole time the trans bucking, and idle bouncing around. I figured I would order an iac in the morning. I went out yesterday morning to order it, and the truck started up, idled high for about a minute (it was dead cold, and was ~0C overnight), then the idle started lowering until it settled at the proper rpm (~650). Took it for a drive, everything running perfect. I decided to go to the store, and give it the restart test. Shut it off, went into store, couple minutes later it started right up without a problem. No issues again.

It's been fine since... So I wish I could tell what fixed it... All I know is that it runs perfectly again :lol:

 

Whenever you remove the IAC, you should disconnect the battery to force the PCM to relearn the IAC position for proper idle. It will happen on its own sooner or later which is what I believe happened here. There are no guarantees that the problem will not resurface if the IAC valve does have a problem, but at least you have narrowed down the source! Hopefully it works out well for you!

 

Well, a few weeks later, and it's acting up again

I had already changed the iac, cleaned the maf, gave in and replaced the maf, gave a complete tuneup, all without a change. So I thought back to when I tested the tps... The truck was warm when I tested it. So I decided to change that as well... Why not? Everything else has been changed! LOL

Jesus H Christ.... Didn't the old tps come apart in pieces in my hand when I took it off the throttle body. The center pivot came right out, and the metal retainer (??) ring fell out. I put it back together, and the thing sounded like a baby's rattle!

Put the new tps in, and of course... it runs perfectly with a cold motor... *sigh*

Oh well, at least my other preventative maintenance is taken care of!

 

So... Is the problem still there after the TPS replacement? Or does it go back to its porr running ways once it heats up?

 

Well, I thought I had the problem beat... It was ok for about 3 days, now its' back to the same scenario...

So.... Up to now I have changed IAC, TPS, MAF and full tuneup.

Thing is, it idles high, if I let it warm up to proper operating temp, shut it off, wait for a couple minutes and start it, it runs perfectly fine afterwards. It only runs fine if I do this. Since I have changed the TPS, the idle doesn't magically/randomly come down while driving.

I'm starting to think maybe the ECT might be a cuplrit? It seems that it's putting too much fuel when it's acting up. This is why I have to wait a couple minutes to start it, it will just crank and sputter if I try right away, and put out puffs of black smoke. Maybe the sensor isn't properly reading until a restart? Or maybe you think and air bubble caught there might do it?

I wish I had a scan tool... I'm almost giving in and taking it to the stealership to get it fixed...