I've been hunting down an issue with my 94 for months now and I'm all out of ideas. I'm hoping somebody has had a similar issue.

The problem. The truck seems to run pretty good and have some good power for a 3.0 but usually after it gets a good heat soak it falls on its face. It'll do good for a long while then out of no where it'll be down on power and around 4k it'll fall hard and even almost slow down. Sometimes I'll feel a little hiccup on heavy accel.

I plugged in the scan tool (snap-on modis) and found its pulling all the timing out. It always shows that it's knocking and pulls it to around only 25 to 30 advance on heavy accel. When it falls it pulls it to an average of 10. When it hiccups it backs it down to 0-3.



Things I've checked:

No codes

Coolant temp is usually 197. I've seen it as high as 204 on a hot day running hard at slow speed.

Exhaust back pressure checked at the o2 bung. No back pressure.

Timing is set at 10 degrees.

Checked EGR and PAIR. Looks to be fully functional.

No vacuum leaks anywhere

Fuel pressure is 45 at idle. 55 on accel and steady

TPS adjustment and function

VAFM function




Things I've tried/changed since it started happening:


Electric fan swap.

Egr and pair delete

Replaced o2 (Denso)

Intake silencer delete.

Cleaned VAFM

Replaced plugs (NGK)

Replaced cap/rotor/wires (Import Direct)

Gumout in fuel and vacuum soak

Running 93 octane (Shell usually)


That's all I can think of as of now but I'm sure I check other stuff too. The only mod I did before this started happening was cs130 alternator swap back in February way before it started giving me issues. It seems to be worse hot outside

 

Injectors maybe, Knock sensor could even affect the timing, might be getting into a faulty state when it gets hot. Unfortunately, you can't just throw ice on them because they are buried under the intake manifold.

You could however use this method on this link to you advantage for a knock sensor. https://www.yotatech.com/forums/f116...e-52-a-254275/
You could split the small harness leading the OEM sensors that go through a tiny opening near the front of the intake plenum. Solder the harness to the GM sensor, if it still gives you problems you can solder the harness back to the way it was. No harm no foul. I did that on my 3.4 and it never gives me any problem, which is why I'm even recommending it at all.

If it's an injector that quits after it gets hot, then you would have to dig in regardless,

That's my guess, and what I would try if it was mine. I'm no certified mechanic. I'm surprised it's not giving you any codes, maybe use the paperclip trick just to make sure before you do anything.

 

Yeah I've been thinking of a faulty knock sensor. Problem is it always pick up knock and pulls the timing weather it's cold or not. At idle or a real light cruise it doesn't show knock at all and timing could be anywhere from 30 to 40 depending on conditions. Also usually if a knock sensor goes bad it'll create a faulty circuit and set a code rather than set false knock. I know the gm trick and have thought about trying it but haven't had $50 to spare on a new sensor just to see what happens. Everything I've tried so far was either next to free or it needed to be done regardless.

I have a hard time believing it to be injectors. Usually if it's bad enough to lean out to knock on accel you usually feel it miss too. It runs super smooth, idles perfect and o2 readings always show to be rich. I do have a new set of injectors though but I managed to misplace 3 of them. If I manage to find them I'll put em in the same time I do the valve cover gaskets

 

The 3VZE generates a (slow by 21st century standards) serial data stream, and one of the signals is "Knock." I assume that's what you're referring to with "it always shows that it's knocking." Which is normal; the purpose of the knock sensor is to allow the ECU to advance timing until it just starts to knock, then back off. (Code 52 is when the ECU "doesn't hear from" the knock sensor at all, which clearly isn't your problem.)

I don't know what you mean by "pulling all the timing out."

Note that the 3VZE doesn't have an absolute timing sensor. Instead, the "number" the ECU reports for timing is ASSUMING the base timing is set correctly to the sticker value. Have you checked the base timing, using the FSM procedure?

 

Yes as I stated in the first post the base timing is set at 10* according to the FSM. With the data pins disconnected it comes down to 8* at idle like expected. When I say it's pulling all the timing out I mean the ecm is retarding it from where it should be. Lets say under accel it should be around 35 it will be close to 20 when it runs"good" and close to 5 when it falls on its face.

I know the refresh rate of the scan tool on these engines is very slow especially compared to modern obd2 ecm. Unfortunately it's the only thing we have to watch the ecm make timing adjustments on the fly.

In my experience of working on cars. It shouldn't see knock all the time. The ecm should have a set table of timing curves programmed into it that it wants to run under certain conditions. If no knock is detected it'll run that timing. If there is knock detected it will retard it a couple degrees at a time until it no longer sees knock and it will continue to run it a couple degrees more advanced until the knock goes away.

 

Also I've checked the resistance on the distributor coils and they all land well within the FSM specs.

 

I'm only asking because it sounds like you know what you're doing, you notice anything weird when you pulled the old distributor out? If the gear on it is damaged per say and not turning the distributor correctly, and the ecu is trying to compensate for it, then maybe,,,

Then again, we just finished a thread not too long ago, where someone had just about the same symptoms, but it turned out the idler timing pulley exploded and it was causing his motor to knock which made his motor die eventually. WOuld probably make more sense if it's not throwing a sensor code. Just shooting in the dark.

 

I haven't ever removed the distributor. I checked for any excessive play and it seems pretty good. I ohmed the pickup coils which doesn't require it to be removed. The previous owner put a used engine in it right before I got it. He claimed the engine had about 100k and he installed a new belt kit and water pump before he put it in. I don't know how true anything he says is but it for sure did get the engine replaced. The timing cover also has a milage on it (75k) when the belt was replaced while it was in its original vehicle. So if it's true about having 100k on it then the belt isn't all that old. I'll probably pull the timing cover off and check it out but I'm doubtful

 

I have a data logger and have also noticed that under heavy acceleration the timing runs around 15-20 deg. BTDC. When hot it always feels slower but I have not tried to compare data between hot and cold. I would like to plug my data logger into a bone stock engine to see if its the same. Maybe its just the way this ECM works?

 

Perhaps that's just how these are but its kinda odd. I watched it again last night with a new thermostat with a few holes drilled in it. It ran a little on the cold side at 175 I made a few hard pulls on the highway and a few times it felt real good, didn't show knock, and was advanced as high as 45*. Even if 15-25ish is normal I'm still having issues with it pulling it as low as 0-5* and when it does that it still has very little power. Makes me wonder if these engines have a common problem with this and nobody knows because a scan tool that can read it is kinda uncommon. I feel like when it does good it actually has some pretty good power. It still isn't race car but not terrible. Maybe its dubbed 3.slow for other reasons.

 

Maybe Scope will chime in again. He seems pretty knowledgeable regarding these older systems.

 

I figure its time for an update. It still has its issues. I decided to try some colder plugs. I installed some NGK heat range 7. Stock is 5. It seemed to help quite a bit but its not fixed. I pulled the intake a couple days ago and changed the valve cover gaskets. As it turns out the injector grounds were loose as a goose and corroded a bit. I'm not real sure how it even actually ran let alone as smooth as it does. I cleaned it up and tightened it down. Now with the combination of the two it seems almost fixed. It still does it but I gotta drive it hard for a good long while and it doesn't seem to last as long. I'm worried though that when I put a normal stat back in it'll get worse.

 

Another procrastinated update

Well I've decided it's likely just a tired 3.0. It's started to make a little extra bearing chatter. It's very audible while ight on the throttle in higher rpms and cold starts. Makes sense why my power always seemed to drop hard after 3500. I think when it gets ran hard the oil gets hot and thins out. At that point it gets some extra chatter and the knock sensors hear it. I've started running 15w40 and it helps with the noise in higher rpm but that's about it. I've made the decision to stop life support for the 3.0 and start working on a 3.4 swap. Stay tuned for that in my build log when I decide to remember to update it.

 

Mine only ever began to chatter like that when and if there was a cooling problem. For example, if I ever run low on coolant or something it will chatter and be powerless, as it is on the verge of overheating. Over the years that has happened a few times and it indicated that I needed to address something in the cooling system. Five years ago I drained the entire system, flushed it out, filled it with fresh toyota red and a bottle of Subaru coolant conditioner from their dealership and I never had the thing (knock wood) chatter like that again. Doesn't loose a drop of coolant.

Are you sure you do not have a cooling system issue? Even a bubble? My temp sensor on the dash never tells me the true story until it is too late. Anything above 40% from cold and I know to take a look.