neoworm0

This has been an ongoing issue for over a year now and I am at my wits end. Getting to the point where I'll be taking it to the "Stealership" soon.

First off I just changed the plugs and 1,2,4,5 and 6 indicate she is running lean. #3 is fouled and smelled like gas when I pulled it. Truck also pinged like a SOB under load. That's a recent development.

I changed the plugs to NGK iridium's gapped .031". New NGK wire set as well. New cap and rotor. That seems to have solved the pinging, for now.

Now for the real mystery. For the last year the truck seems to have some kind of ignition gremlin. Hard to explain but I'll try my best.

Typical scenario: Truck sits overnight in the garage, start it up in the morning, runs fine. Drive to Starbucks. lets say 10 min. Start it up again, runs fine for a few seconds or even minutes and then tach just dives and the engine ALMOST dies, then right back up again. Does it a few more times and eventually dies.

It only actually started dieing completely recently. Before it would always recover and run fine. If you turn the ignition off and restart, it will USUALLY run just fine after that. Sometimes I have a repeat performance but not always.

Seems to be most common after short trips but not always. Also seems to be most prevalant when the truck is warmed up, HOWEVER it has happened cold.
AND it doesn't throw any codes.
That is what is confusing me, there seems to be no real pattern. Sometimes it will be fine for a week or more and then for awhile it happens daily. It is slowly getting worse too. Anyway here is what I have checked so far.

TPS = fine
MAF = fine
Coolant temp. Sensor = fine
O2 sensor = fine
Igniter = resistances good
Coil = primary and secondary resitances good
Distributor = good
No vacuum leaks - at least so far as I can determine.

Truck has had Mobil 1 changed every 3000mi religiously (overkill I know but I figure it's cheap insurance) and chevron 89 gas (92 lately cause of the pinging).

So far the ONLY thing that has actually made a difference, a mechanic friend suggested it was the throttle body so I replaced it with another used one. It went away for about 2 months but returned.

Well if you actually read this whole thing I appreciate it and any suggestions would be welcome before I take it to the dealer and get raped.

TIA

ozziesironmanoffroad

try some throttle body cleaner, clean the MAF, and double check your vacuum lines. also try replacing the o2 sensor. i know, i know, you checked them, but sensors can fail just enuf to screw stuff up, but not throw a code. sounds like an air/fuel delivery problem. try adjusting your air to fuel ratio.

neoworm0

Thanks for the advice. I've cleaned the throtle body several times already. No difference.

I've gone as far as replacing the entire MAF. No difference.

Vacuum lines have been quadruple checked but I'll check them again.

I'm reluctant to just change the O2 sensor. I just checked it tonight per the FSM and it checks out perfect. It's not exactly cheap. Might have to though.

motoriderbc

Does the tach drop to zero or is it just responding to the stumble?

TNRabbit

Have you tried flexing the wiring harness in different locations? Sounds like you might have a short somewhere...

thook

Might have to start testing connections at the ECU harness.

Interesting that the only thing that's made any difference was replacing the TB and then returns. I'd guess that whatever the problem it's affecting the TB. Was the old one gummed up?

Try testing your fuel pump relay switch...
http://personal.utulsa.edu/~nathan-b...96circuito.pdf
The fuel pressure regulator...
Just pull the vacuum line from it and look for fuel. If there's fuel in it, it's bad. If not, then hook a long vac line to it, and suck on it. It should open and then shut when you release.
You also try jumping the terminals FP and B+ (fuel pump and battery.....in the diagnostic box), drive it around a bit, and see if that has any effect.

neoworm0

Tried flexing the harness. No difference.

Tach drops to zero.

The old TB was pretty clean. So is the current one.

I will try testing the fuel pump relay switch and fuel pressure regulator tonight after work. I was going to start testing the voltages at the ECU last night but realized my meter probes won't fit into the back of the connector. I'll have to remedy that.

Thanks guys.

SEAIRESCUE

STICKING EGR..plug vacuum line to it and see what happens. The older ones do not throw a code. Its also located close to #3. Also, a bad O2 sensor will put things into a lean as well as rich condition.

rdharper

Has the problem as you describe it, become more frequent over the last six months?

motoriderbc

Since the Tach drops out, it sounds like a primary ignition fault (ignitor, distributer pick ups) Resistance checks are useless, they have to be checked running. You need a labscope or a graphing DVOM to really see whats going on. Your probably going to have to take it to shop that has a good driveability tech (personnaly I'd avoid the dealer)

neoworm0

Yes.

I would like to avoid the dealer too. My only thought was that maybe they've seen the problem before. Any suggestions on where to take it?

rdharper

When you say you've replaced the MAF... there is no MAF on this engine, it is a VAFM.

MAF is mass air flow
VAFM is Volume air flow meter

They do the same thing, but are totally different in the way they accomplish it.

The VAFM wears out... exactly like a record player will wear out. There is an arm that sweeps over a phenolic material. The pickup head on that arm will eventially wear a groove in the phenolic. The unit feeds back the angular displacement of the air gate in the intake. The initial signs of excessive wear will be intermittent behavior as the ECU is getting wrong information about the air gates position.. intermittently.

If that is the problem, it will get bad enough that you will be lucky to get across the street without the truck cutting out, and bucking on you.

This can be hard to detect with the resistance tests as prescribed in the FSM, at least in the early stages of failure.

The VAFM wore out on my '85 at about 200k miles. I took off the silicone cover with a pen knife, after which it was obvious what was wrong. I tried to repair the material several ways.... thought I'd fixed it several times, probably getting another 5k miles out of it. Eventually I replaced the unit with a new one.. not cheap.. btw.

neoworm0

You're right. Got the terminology wrong. Regardless I replaced the VAFM, albeit with another used one. That didn't make any difference and I think it unlikely that I would have been unlucky enough to get one that had the same problem as my original. Not impossible I suppose. However the truck it came from ran fine.

I managed to get another distributor for $20 from someone on another board. So tonight I replaced the distributor to see if that makes any difference. I figured for $20 it's worth a try. I've been looking for an excuse to buy a timing light anyway.

I drove around a bit and it SEEMS to run better but that probably is wishful thinking. Pinging is totally gone though. Didn't die on me after restart but I won't really know if it's fixed for a day or two. It seems to do it pretty much daily now so if it doesn't do it tomorrow, I'm optomistic. I didn't get a chance to test the fuel pump relay and regulator or the EGR.

Is there anyway to check the VAFM without taking it apart, other than the resistance checks? I assume not otherwise you probably would have done that, eh.

Hopefully I won't have to now. By the way I appreciate the help. Thanks again.

Guardian_Saint

Water temp. sensor problem, ECU detect it, sometime coolant not enough to verified temp. ECU thinking, causing lean? Open the heater hose and pour some coolant or water until its fill up back there and try from there, just my $0.02 cents. I'm guessing though.

neoworm0

Coolant temp. sensor was my first thought too. If it comes back again I will look at that again.

neoworm0

Well it ran fine today. No problems. I wont' declare victory for a day or two yet but I've got my fingers crossed. Still have to figure out why it is running lean though. I don't think they are related.

motoriderbc

Hope the distributor works for you. Why do you think it's lean? If its because the plugs are "white" thats normal for an injected engine with an O2 sensor.

neoworm0

Well I took it out again tonight and it is running better and the issue hasn't returned so it looks like it was the Dist. despite everything checking out on it OK. We'll see though.

Ya exactly plugs super white. I always thought that was lean. Didn't realize that was normal. Never really noticed before I guess. So I guess it is nothing to worry about. Other than the fouled and wet #3. i'm going to check my EGR tomorrow since I'm into this now.

neoworm0

Well it wasn't the distributor. It's back with a vengeance. Checked the EGR system today, everything checks out fine. I stopped there. I'm sick of this . Taking it to a shop this week. Some things are just worth paying someone else to do.

Of course I'll probably change my mind when I get the bill.

I'll post up the results when and if they find something in case it may be useful to anyone else.

Bigblock

Do you have a big jailer key ring on your ignition key 10 lbs of stuff hanging from it?I've seen this wear out a few switches.