94 3.2L suddenly running really bad

Ok, my turn to get some help. Was driving my Rodeo when it suddenly lost power and started running really bad. Engine died on me several times while trying to get it back home and it had very little power. It's running like ignition timing is out of whack. Timing belt is fine and marks still line up properly.

No check engine light, no trouble codes. The only things I can think of that effect ignition timing are the MAP sensor, checks out fine and the crank position sensor. Can't find it. Anyone know where it is? Under the lower timing cover, perhaps?

I put a timing light on it and its very erratic and usually to far advanced. While idling goes as much as 20 degrees BTC and the mark swings up and down +/- 5 degrees almost all the time.

Need help from the real techies here. Thanks.

Have Exhaust Back Pressure Checked At An Exhaust Shop May Be A Cat

Tensioner holding while running? CKP is on the side of the block.
Is it possible a coil pack quit? Did you pull the plugs? I had no cel when one of mine failed.

The tensioner is good. Just replaced that not too long ago and it's still holding. I'm more inclined to thing coil pack than clogged cat. I guess I need to get me some chicken pliers. :icon16: Or use the timing light as a spark tester.

I had a problem like yours. Turned out to be bad connection at the fuel pump. I live in Western NY and lot's of road salt. My CEL came on though and it put a O2 sensor code as the problem. It was however a bad connection at the fuel pump all along. Hope this helps...

Coils check out. Pulled the spark plugs, nothing unusual there but replaced them anyway. Seems to run better but something still is not right. I decided to put it off to replace the intake manifold gaskets on my wifes sisters 03 Grand Am. Stupid GM and their stupid plastic gaskets. We're making due with one car ATM.

before discounting 2eye...'s diagnosis of plugged cat causing excessive backpressure, because he is an isuzu mechanic that sees alot of vehicles in a week -- hey 2eye --- if you would explain, how would too much backpressure cause the problem this guy is having? thanks...

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now, that being said, some of your other efforts sound familiar to me when i had an ignition coil problem that caused all sorts of horrible running. the (3) coils and ignition module on my 1992 trooper are the same as on yours.

i too used the inductive timing light as a diagnostic tool and got some weird results that were not entirely consistent. i ended up buying a cheapo ignition spark short tester and ran it up and down the wires and eventually got an intermittent hit at one of the coils. i got some easier visual confirmation by checking it at night and the sparking was more apparent. the problem was more pronounced as things warmed up.

it turned out that one of the coils was bad and intermittently doing a short number and jumping ignition wires, which with the double duty firing of each coil caused the weird firing patterns. i ended up replacing the whole ignition-module-ignition-coil-assembly as a unit, but could have, in retrospect, just replaced one of the coils if i had the parts sitting on the shelf to test with.

so while waiting on 2eye's further help, you might want to trust your instincts here and continue to focus on the ignition system.

//bc




Coils check out. Pulled the spark plugs, nothing unusual there but replaced them anyway. Seems to run better but something still is not right. I decided to put it off to replace the intake manifold gaskets on my wifes sisters 03 Grand Am. Stupid GM and their stupid plastic gaskets. We're making due with one car ATM.

I've seen cars with plugged cats and this doesn't seem like a plugged cat to me. The bad performance was sudden, like flipping a switch and now it runs better than it did before.

Well, I've pretty much ruled out spark plugs, plug wires, MAP sensor and fuel pump. Fuel pressure is steady and I left the gauge attached while I test drove. Car acted up, died and I checked the gauge and still had fuel pressure. New plugs and wires.

When it died today and as I was trying to restart it, it cranked very rapidly just like when the ignition timing is messed up. It finally restarted and I got it back home. I'm still suspecting the crank position sensor but I sill can't find the dang thing. Anyone have a SPECIFIC location for it? I know it's in the lower portion of the block somewhere but all the 4x4 gear is covering it up. Maybe even a pin-out of the ECM would help and I could just hook-up my meter there instead of digging.

Also, any way to hook up a laptop to the ECM and log the data in real time?

Back Pressure Is Checked Between Exhaust Manifold And Cat If The Cat Is Plugged It Results In High Pressure In The Exhaust System Because There Is No Place For The Exhaust To Go Which In Turn Will Reduce Performance Or Possibly Not Allow The Vehicle To Be Able To Run At All

Could a bad cat give intermittent symptoms? I wouldn't think so but I could be wrong. Seems like it runs ok until I've driven for a few minutes. I'll drive it some more to see if I can notice a pattern. It just doesn't feel like a back pressure problem. Seems more ignition or emission control related.

Figured out where the CKP connects to the ignition module. Now, my Chilton manual only mentions the CKP for 1996 Rodeos and says the cranking voltage should be 2.5 volts. I wouldn't think this would be much different for 94 but I'm not getting anywhere near that voltage reading. With the engine running I'm getting less than 600 millivolts at higher engine speeds and less than 200millivolts at idle. I can't find a spec anywhere for my 94.

I'm probably off track here since usually if the voltage coming from the CKP is too low, the engine just won't run.


Also, when the car is acting up, the exhaust sounds different at the tail pipe. Sort of puffing or something. Hard the describe. The thing is idling normally ATM.

New information:

It sat here in the garage idling for about 45 minutes with me poking and testing it, revving the engine and there seemed to be nothing wrong. I went to back it out to drive it again and the engine shook just slightly, I pulled back into the garage and it started shuddering and then croaked. Now, it's sitting here idling normally again and now my voltage reading from the CKP is under 100 millivolts at idle. If I really rev the thing up, I can almost get the CKP output to 700 millivolts.

i'll try to look up that stuff on the position sensor...

but the more you write, the more it sounds like what i had with the coil shorting out...including the difference after it ran for a while.

did you do any testing/diagnosis and/or visual checks at night?

//bc

i'll try to look up that stuff on the position sensor...

but the more you write, the more it sounds like what i had with the coil shorting out...including the difference after it ran for a while.

did you do any testing/diagnosis and/or visual checks at night?

//bc


Yeah, I looked under the hood last night. Couldn't see any arcing.

Thinkin Of Somtin I Once Had One With Same Symptons And Would Also Die Out At Times
Found That The Ckp Wiring Was Caught Up Under The Engine Mount And Was Intermittantly Shorting Between The Mount And Frame

like most input components for the computer, the reference voltage *to* the sensor should be ~5 volts. and you should get a reading *from* the sensor (to the computer) of ~2.5 volts when cranking. first test for reference volts, then output.

sounds like the short idea might be it, doesn't it...

//bc

like most input components for the computer, the reference voltage *to* the sensor should be ~5 volts. and you should get a reading *from* the sensor (to the computer) of ~2.5 volts when cranking. first test for reference volts, then output.

sounds like the short idea might be it, doesn't it...

//bc


Doesn't the CKP induce its own voltage? I don't think there is a reference voltage. Also, the CKP doesn't connect to the ECM, it connects to the ignition module.

Doesn't the CKP induce its own voltage? I don't think there is a reference voltage. Also, the CKP doesn't connect to the ECM, it connects to the ignition module.

it's true that info came from a newer, later application; but the idea is the same, the ignition module gets the reference voltage from the ecm and the crank sensor gets it from the dis module. the test for reference and output voltage should be instructive.

however, from what what you've said before and what i suggested concerning the ignition module and coils still apply. there can be, and often is, a difference in under-load and idle conditions. i would suggest attempting to isolate the problem to a particular coil by observing either the timing light or by disconnecting spark plug connectors and observing the spark across a gap to ground with the engine reving. of course if you had a spark tester, it'd be alot easier and surer.

by either method, if you find a problem wire (probably two because each coil powers two cylinders), then you can swap coils, they are interchangeable, and see if the problem follows.

//bc

I am clueless about diagnosing electrical problems, but the Haynes I have says that reference voltage is 5 volts.

I am clueless about diagnosing electrical problems, but the Haynes I have says that reference voltage is 5 volts.


I no longer trust my Haynes/Chilton regarding sensor readings. I thought I had nailed the problem with a bad MAP sensor but it turns out that the repair manual is giving the wrong voltage readings for a good sensor.

I can safely say that it isn't a fuel delivery issue. Hooked up a fuel pressure gauge and drove it till it died again and still had fuel pressure.

Anyway, back to scratching my head on this.

Ok, a little update. I went out and bought 6 inline spark testers with the lights in them. Hooked one to each spark plug wire at the coil packs and then drove around and waited for it to croak. Seems like it takes longer and longer to screw up but when it finally did stall, I popped the hood and tried to restart. Took a lot of cranking to get it to restart but I had good spark at all six cylinders and good fuel pressure while I was cranking it. Now, I'm even more baffled than before. The only thing I can think of now is I may have a fuel injector or two trying to crap out.

Anyone have any ideas now? :icon16:

How did this one turn out... what was the problem? I've got the exact behavior on my '94 Rodeo and haven't got a clue. Thanks.

I never did find the problem. It just went away on it's own. I can only guess that I got a bad tank of fuel. Once that fuel was gone, it started running normally again.