VegasNaturist

Okay, Let me see if I can accurately explain what my truck is doing. It is similar to an idle surge problem described in many threads, but opposite. This is the second occassion where my truck (described below) has had this issue. I can throttle the engine up to about 1,200 rpm, then it cuts out and drops to idle, rev up, drop to idle, etc. It will do this both under load and free-wheeling. I can feather the throttle to nurse the truck a short distance, but I have to keep under that "trigger" rpm. The truck idles fine and runs fine as long as I stay under that rpm. If I turn the truck off and restart, the problem goes away temporarily. Sometimes for just a few miles, other times for several weeks.

I checked the VAF sensor and just above part throttle, the ohms went goofy. Thought yea and replaced it, Truck ran fine for several weeks so I thought I had it licked. It returned this past weekend.

I ohmed out the TPS and it is adjusted correctly, and the readings are within spec all the way to WOT.

Checked for vacuum leaks, none found. Checked for good bat & neg connections at battery, body, harness, engine, etc.

Thought maybe my brake lights were sticking, causing the ECM to cut fuel, but I can't duplicate the problem by stepping on the gas and brake at the same time.

There are no codes...

Anywhere else I can look?

John

Edited to add:

Also, to clarify, the truck has about 5,000 miles on a short block and less than 500 miles on a new head. Head was replaced between occurrances and because of a stripped plug hole.

Don't know if this is relevant or not; but, both times it started doing this, we were off road and at about 5,000ft elevation. We were not running hard, just some semi-washboard roads and reasonable speed for the condition. Checked for loose connections everywhere and haven't found any.

yayfortrees

Wow, you've done a great job covering all the potential culprits. Much better than most that I've seen on here.

I don't know if this will be of any help, but have you tested your temp. sensor? It's a longshot, but you've already ruled out most other items.

Also, have you changed out your fuse box power supply wire and body ground? Not likely to be the cause of your problem, but it's a good cheap upgrade that I think should be done anyway, so it can't hurt.

Just to throw out some other ideas: Have you checked and gapped all the spark plugs? Resistance check plug and coil wires?

Off topic: Do you live close to Eastern Ave.? I used to live with some friends in the Equestrian Apts. south of St. Rose Parkway on Eastern. Had to change my clutch pedal bracket and slave cylinder in the complex parking area, even though you weren't suppose to make any vehicle repairs.

VegasNaturist

Thanks, I try to use the search function and research as much as I can. If I'm still stumped, then I ask.

Haven't checked that yet. The dash gauge seems to read accurately, so I figured... I did print the ohm's table, perhaps I ohm it out if just to eliminate it.

Replaced? No. Cleaned and resecured? Yes. Good point though...

Yes, all good.

Actually, not far from there. Sunset and Green Valley area. I pass by Eastern and St Rose on my way to work. I'm a mechanic at the school bus yard at LVB and St Rose.

Thanks for the suggestions...

John

bikedave99

John, just to be clear, the temperature gauge in your dash gets its signal from a different sending unit then the ECU. The gauge sender is located on the passenger side of the engine low down on the block and can be seen through the wheel well. The temperature sensor for the ECU higher up and kinda in front of and below the throttle body. Two plugs right next to each other and its the one on the right if you are facing it. This is all assuming a 22re.

All that said I have had the same problems you have had and have not figured it out either so we have a mutual interest. It is quite difficult to diagnose something so sporatic as this. Like you said it normally will reset itself if you shut off the engine and start it again. I did find that while mine was having this issue I left it idling and instead of shutting it off to "reset" i unplugged the MAF and plugged it back in and it seemed to "reset". I checked my MAF and it was within spec but I have heard they can fail and still appear to be functional. The other thing I will be checking is my fuel pump.

VegasNaturist

Thought it might be different sending units, wasn't sure. I know which one you're talking about. Next to the cold start injector time switch.

Well, I changed my VAF sensor. Problem went away for quite a while. Hmmm. Have to check that out. I'll keep you posted. We bought the truck to use to explore the sorrounding desert and ghost towns, so I need it right. Don't want to be stranded out in the middle of nowhere!

John