Computer issues??

[Spitfire1975]

So...my '86 2.9 5 speed has recently developed an issue I need to resolve. I have not yet taken the time to pull codes, etc... but this truck has had dang near every sensor replaced, compression is good, vacuum is good, exhaust is all new (including cats), fuel pressure regulator and high pressure pump replaced, no found vac leaks, all intake gaskets are new, ignition components replaced, etc.... It's been worked on....

At times, while driving, it will begin to either a) buck and jerk like it is running out of gas or b) go into a mode of high idle rpm's (1500-2000 at idle). If I turn the truck off and immediately restart, the problem goes away and stays away anywhere from a few hours to a few days.

Any suggestions before I start the tedious hunt and peck search......again? I am beginning to wonder if the computer is needing a replacement.

 

[Shran]

Do you have an automatic or manual trans? A sticking IAC could cause that. I would see if you can borrow an ECM from somewhere to swap in for a couple days before you buy one, but I am guessing that's not the issue, although it is possible.

I might have a spare ECM that I'd let go real cheap, will have to go look.

 

[Spitfire1975]

It is a manual. The IAC was cleaned out not that long ago , but I will do it again. This is the second ECM I have put in it. The original went bad and I could not for the life of me get the thing to run right. Poor idle, no power, etc... This one I put in was a junkyard find and is God-knows-how old, but it was working.

Let me know about your ECM. If I can't track down a bad ground, sensor, IAC, etc.... I would be interested.

 

[Spitfire1975]

So, I have been dealing with this intermittent high idle/pig rich issue for a while now. In fact, it is getting worse. I unplugged the IAC to see if it would die, and it did. I took it off anyway to see if it was dirty and it wasn't. It seemed to activate the plunger fine when charged. Pulled codes and this is what I got:

Engine off:

23; pause-one flash, then 54, 63, 33, 53

Engine running:

nothing at all; pause-one flash, then 25 (knock sensor), 23, 13, 77 (goose test)

As for the TPS codes, If I adjust the TPS (which is 7,000 miles new) to the proper voltage, the RPM's run about 2000. The voltage is about .6-ish to keep the idle about 1200 RPM's.

I changed the IAC motor with one I had in the garage and it made no difference in how the truck runs. With THIS motor, when I unplugged the connection the truck di not die. I assume it is no good.

I am getting perplexed. Do I have a screwed up (or two) IAC? a lower intake air leak?
a bad MAP (I changed that out with a spare as well. No change.)? Back to a bad ECM?

I haven't done a recent compression or vacuum test (~5,000 miles ago). Last time the numbers looked great.

?????? Help?

 

[adsm08]

Check your timing.

Check the wiring for the knock sensor.

Adjust TPS to correct voltage.

Then get the truck to temp. Run it at 2000 RPM for 2 minutes. Unplug IAC, block off PCV, and then adjust the hard stop screw on the throttle until the desired engine speed is achieved. 700 RPM for auto, 850 for manual. Restore all vacuum and electrical connections and retest for proper operation.

 

[Spitfire1975]

Will do. Probably not until Tuesday, I am going to be gone.

I forgot to mention that I reset the timing earlier, it was at 4*, got it set to 10*. And yes, I remembered the spout.

What am I looking for when it comes to the knock sensor wiring? Just general corrosion, etc??

 

[adsm08]

What am I looking for when it comes to the knock sensor wiring? Just general corrosion, etc??

No, unplug the sensor, unplug the computer, and do resistance tests on the wiring. You are looking for a break. Should have .5 ohms or less.

 

[Spitfire1975]

Ok. I tried again what I have done before, blocking off the throttle body and choking off the air. It has always killed the motor.

Bingo.

Big ass intake manifold leak back by the distributor. Which kinda pisses me off because I have changed that thing before. I must have done something wrong.

What gets me is I have looked for a leak there many times and have never found it.

 

[Spitfire1975]

Got all the intake and upper gaskets replaced a couple hours ago. So far, idle is steady and 850 rpm.

Damn vacuum leaks.

 

[Spitfire1975]

WTF????

How long does the black rtv silicone used in sealing the intake manifold need to cure before starting the motor? It is still leaking back there. Idle is better, but still goes between 900-1200. Jerky and hesitating while driving, not so much on acceleration.

Would a bad head gasket cause an air leak like that, or am I too much of an idiot to figure out how to properly install a freaking gasket. What else would cause a leak in the back half of the engine?

 

[adsm08]

It should have been dried enough in the time between your posts. I don't use that stuff though. The first time I replaced my upper intake gasket I did it dry with no issues.

The second time the intake had been powder coated. I used the copper spray.

 

[wdennie]

still having problems could have a warped or cracked intake that only shows itself when hot

 

[Spitfire1975]

still having problems could have a warped or cracked intake that only shows itself when hot

So....

How could that have happened? The truck has not overheated. Is it common to have the intake warp?

How do I find out if this is the case?

 

[Spitfire1975]

I replaced it again, waited well over a day for the silicone to "cure." Seemed good. Until it ran for 20 minutes. Back to the same issue, so I am going to assume the intake is pretty well screwed.

I am tempted to part it out and go get an Explorer.

Before I go buy a new manifold, I ask this again....could a bad head gasket cause this kind of leak? When I cup my hand over the throttle body, it whistles like a son of a bitch from somewhere back by the distributor. I don't want to buy a manifold only to have to do the heads.

I guess I will be doing a compression test tonight.

 

[Pbheadshots]

4.0 swap!

 

[adsm08]

A head or gasket would be very very unlikely to cause that kind of issue. Not without more serious issues going along with it. A compression test would show the problem if it is in the heads or gasket though.

 

[Spitfire1975]

A head or gasket would be very very unlikely to cause that kind of issue. Not without more serious issues going along with it. A compression test would show the problem if it is in the heads or gasket though.

That's kind of what I figured, but wasn't really sure.

I let it cool all night and am about to restart it. If the idle is good and then goes bad while hot, I will run the compression check and if it comes out good I will start looking for a manifold.

Ebay has a couple. Is this an item I should be buying new? If so, where is a good place to find one? Rockauto doesn't seem to carry them.

Thanks for your help on this.

 

[Spitfire1975]

Finally just changed the manifold and it seems to be doing ok. We'll see how it holds up.

 

[baggedcereals]

Has the new Intake fixed your problem? After reading your postes this is almost how my truck acts!

 

[Spitfire1975]

nah.

It did solve the intake leak problem, but it still runs weird. One day it is great, the next it is bucking and jerking with an inconsistent idle. Seems to be missing a cylinder pretty regularly. I am starting to think it is just a messed up ground somewhere, or the in tank fuel pump going bad. It seems to act up more when it is warm outside, but all of the temperature related sensors have been changed.

It is still a big mystery.