Low Idle - TPS? Timing? Plugs/Wires? Fuel system?

[gamefreakgc]

Now I'm incredibly confused. I've got a rebuilt ECU from a reputable vendor, OEM IAC valve and OEM TPS. I've replaced the fuel pump, fuel filter, crank and cam sensors (both Denso), ECU coolant temp sensor, and the cams and crank all line up to TDC. It won't fire up, even with starting fluid spray. If the truck sits for a day or two, it seems like it will try to fire up for about 1 second then quits.

Still cranks but won't start, and the IAC valve is cycling open and closed really fast when the key is turned to "ON" (engine OFF). Pulling the EFI relay under the hood stops that, but the cycling happens even with a known good relay. Pulling the circuit open relay beneath the fuse box under the dash has no effect, still get the IAC cycling.

I've tried disconnecting sensors/parts to see if something is shorted out, each one individually: igniter, fuel pump, MAF, vapor pressure, ABS, cruise module, TPS, and I still get the IAC cycling with ignition ON engine OFF. I've checked every fuse, and they all appear intact (though I haven't checked them with a meter).

And for some reason, I can't get an OBD reader to connect to the ECU anymore (the original or the new one I recently put in), though the original used to work.

Any ideas would be most welcome! :noidea:

It may be time for a very thorough wire inspection. Follow each sensor's wire loom back to the firewall and check for any sort of abnormal bends, old splices (you'll often see the connector still there) or blackening or splitting. After that, try opening up the ECU body itself. It's not hard to do and it's not all that fragile. Look for any damaged capacitors or burn marks. If you have a buddy with a spare ECU, try swapping it out.

You've covered all the big stuff but the inability to connect to the ECU is a big clue here.

Lastly, disconnect the battery ground to fender, passenger's side cylinder head to firewall and engine bay OBD-II port to intake plenum grounds, clean, and replace. Bad grounds do funny things.

 

[19963.4lsr5]

Do you have power at your diagnostic port?


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[Bad Luck]

Do you have power at your diagnostic port?


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Put a test light between terminals 6 and 14 of the OBD connector to see if you have power and ground with the key on. The EFI relay/fuse powers the OBD port and the relay is controlled by the ECM.

 

[TimGFL]

It may be time for a very thorough wire inspection. Follow each sensor's wire loom back to the firewall and check for any sort of abnormal bends, old splices (you'll often see the connector still there) or blackening or splitting. After that, try opening up the ECU body itself. It's not hard to do and it's not all that fragile. Look for any damaged capacitors or burn marks. If you have a buddy with a spare ECU, try swapping it out.

You've covered all the big stuff but the inability to connect to the ECU is a big clue here.

Lastly, disconnect the battery ground to fender, passenger's side cylinder head to firewall and engine bay OBD-II port to intake plenum grounds, clean, and replace. Bad grounds do funny things.

I did purchase a reman ECU, because the original one in there did have a leaking capacitor that buggered up several traces. I also took your advice and cleaned up the grounds, and I am able to get it started up momentarily with a blast of starting fluid now. PROGRESS! LOL **Edit: I should qualify this, that I do have to depress the throttle somewhat to get it to run... not wide open, but to hold it at about 1500rpm

Definitely some weird electrical things going on here...

 

[TimGFL]

Do you have power at your diagnostic port?


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I hadn't thought to check, so I just did now that you mentioned it, and... I do have power at the +B terminal of the diagnostic port, when I turn the key to "on" as Bad Luck mentioned.

 

[TimGFL]

Put a test light between terminals 6 and 14 of the OBD connector to see if you have power and ground with the key on. The EFI relay/fuse powers the OBD port and the relay is controlled by the ECM.

I will check that as well. I do get +B power at the diagnostic port with the key "on"

 

[TimGFL]

So, continuing down this long, winding journey... I cleaned up the grounds as gamefreak advised. And here's a strange (to me) condition that is new.

If I disconnect the fuel pump from under the back seat, and I give a 2-3 second squirt of starting fluid behind the air filter, and give it enough throttle to hold at about 1500-1700rpm, it will start up and run (kind of rough, but it runs) for about 2 minutes (yes, minutes)... during that time, I get white smoke but it doesn't smell like oil or antifreeze being consumed. I am guessing that for some reason the cylinders are being flooded by fuel, and the starting fluid helps combust just enough that it will fire and consume a little bit of the liquid fuel, and once the liquid fuel burns off with the help of the starting fluid, it stops firing.

Once that comes to a stop (I'm guessing it's just running off residual pressure from the fuel line and the starting fluid helps to combust the fuel), I can give a squirt of starting fluid, and it will run for about 2-3 seconds, and during those 2-3 seconds it runs clean and smooth, what I would call "normal."

If I then reconnect the fuel pump, it runs for a few seconds then quits.

I can repeat this process: disconnect FP, squirt, runs for 2-3 minutes with white smoke, reconnect FP, won't run.

To me, it makes no sense. Can the fuel pump create too MUCH pressure and flood the cylinders?? Hopefully this might spark someone else's more experienced mind and point me in the right direction.

**Edit: just for reference, during experimentation today I cracked the fuel line at the tank side of the filter and ran about half a gallon out into a clear container, thinking maybe I just got some bad fuel last time I got gas. It's clean and clear golden gas color, no visible separation or settling, and doesn't smell off or soured.

 

[19963.4lsr5]

Will it start and stay running with ether and the fuel pump relay.

You may just need to hold the throttle open and get it running and let it relearn it’s adaptives.

If you can, get it running and hold it at 1500rpm for a little. Then slowly back off the throttle and let it relearn its idle position.


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[TimGFL]

Will it start and stay running with ether and the fuel pump relay.

You may just need to hold the throttle open and get it running and let it relearn it’s adaptives.

If you can, get it running and hold it at 1500rpm for a little. Then slowly back off the throttle and let it relearn its idle position.


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I was able to get it running on starting spray at 1500rpm for 2-3 minutes, but now I've got bigger problems. Looks like I'm getting coolant from behind the timing cover. Guess it's time to do the timing belt, pulleys & water pump job and/or head gasket. With 280K+ miles (100K that I put on it), I guess it's earned it

 

[19963.4lsr5]

I was able to get it running on starting spray at 1500rpm for 2-3 minutes, but now I've got bigger problems. Looks like I'm getting coolant from behind the timing cover. Guess it's time to do the timing belt, pulleys & water pump job and/or head gasket. With 280K+ miles (100K that I put on it), I guess it's earned it

You can pull the exhaust studs out of the head and not need to remove the exhaust manifolds to pull the heads.


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[gamefreakgc]

I was able to get it running on starting spray at 1500rpm for 2-3 minutes, but now I've got bigger problems. Looks like I'm getting coolant from behind the timing cover. Guess it's time to do the timing belt, pulleys & water pump job and/or head gasket. With 280K+ miles (100K that I put on it), I guess it's earned it

If the head gasket leak is bad enough it could cause a very bad idle. White smoke says that the coolant is getting into the engine and burning off a a rapid rate. Water at idle will quench combustion and cause it to stall.

Bummer this is turning into a massive project but it is repairable.

 

[sag020]

I created an account to reply on this thread.

I had a similar issue. 1999 355k miles gen 3. It would splutter and hesitate, foot to the floor and it barely picked up speed. Like it was misfiring without the misfire. The transmission wasn't slipping, it just wouldn't go.

I installed new plugs & leads, one (cyl #1) was deteriorated so bad it was crumbled up under the connection, I thought for sure that was the issue. I have no idea how the 4R was still running with that connection. No, still done it. I noticed a few hoses were cracked and falling apart, replaced them. No difference. Cleaned the throttle body out, nothing. Took the MAF sensor out and cleaned it, made no difference. I bought a new fuel filter and injector cleaner, still didn't fix the issue.
I plugged in my computer just to see because it never threw a code. I was messing around and found when I put air fuel ratio at +25% it ran fine, I took it around the neighborhood and it ran great. when I released that setpoint to control normally it ran like garbage. I was able to toggle that on & off as I was driving all over the neighborhood with repeatable results. I figured it's running very rich, when I lean it out it runs perfect. It's running rich because either ECU is bad, injector messed up, or bad MAF sensor, IAC valve or a really bad air leak. I can't do anything about a bad ECU except replace it, MAF sensor could be bad, I took off hoses and pressure tested them, they weren't too bad, IAC valve is responding to my commands so it probably isn't that, bad injectors would throw a misfire code so I figured it was probably MAF sensor.... then I stumbled on this thread and some people said to clean the IAC valve. I figured I'd give it a try before ordering a new MAF.

I took the TB assembly apart and took the IAC valve completely apart, scraped a little black gunk out with a screwdriver, filled it with SD20, then blasted it with brake cleaner, then wiped it all down with Chanteclair then drenched it in WD40. It was dirty but didn't look like it was dirty enough to prevent it working properly. I put it all back together and bingo! Runs perfectly and has been for a few days. I can't believe that was the issue.
Just adding my experience so it might help someone else out in the future.