94 PCM questions

[MattStrike]

Well it could be low compression or a jumped timing chain, and I don't trust a volt meter to tell me if a hall-effect switch is working either - especially one that pulses on the order of 30 times per second at idle (600 rpm = 10 rps = 30 pulses/second at the crank sensor 3x interrupt = 30Hz, you would need an oscilloscope to know if it's indeed working correctly). A timing light will tell you if you have an issue with timing. You will have to use a silver sharpie and mark top dead center of piston #1 on the balancer relative to the sub-frame, then mark 20 degrees and -20 degrees advance to know for sure, then the timing light would tell you if you had timing off.

But, I've yet to see a 3.8l that needed such involved testing... Out of curiosity what is the voltage at the battery cold? How many miles on your engine? When you are disabling spark, how is that done? Are the fuel injectors clogged? Lean burn or weak spark or low compression can cause the symptom you describe where the misfire goes away once warmed up.

Do you have the capability of making a video or sound clip of the hard start?

[1oldman]

I believe you can see the pulse from the ICM with one of the better diagnostic code readers also. You may need to take it to a local shop. The big ones like Firestone charge $99, but the independent shop I use sometimes use to charge just $35. - BC

[aj_94ssei]

Battery voltage is 12.7 cold, pulled plug wires off of plugs to see if that made it easier to crank and it cranked over as it should. Not sure about injectors I did do a leak test by removing injectors and pressurizing the system no leaks, the car has 160,000 mi. with a reman charger about 20,000 mi. ago, not sure about how to do a sound clip but I will try. If I can get it to start I am going to get her to a shop this week only the 2nd. time ever I will have let somebody else work on my junk.

[MattStrike]

So, just to verify, that the wires are in the correct order, I can't think of very many issues that would cause it to crank slow other than reversed wires.

Looking from the front of the car at the motor:

.2...4...6
1...3...5.

The coils are in pairs 1/4, 2/5, 3/6, viewed from the front at the coils:
(type-2 delco)
3/6
2/5
1/4

(type-1 magnavox)
6
3
5
2
4
1

[aj_94ssei]

Shouldn’t it be
6/3
2/5
4/1

[MattStrike]

In these engines it really doesn't matter as long as the pairs are together on the coil because they use waste spark distribution. The only reason to do it the way I listed is to keep the wires neat and clutter free and that they don't cross touching each other. Make wires run parallel with a minimum 1/4" or so gap between any two wires (to prevent arcing).

I've done it before, lost track of which wire I have in my hand while working on the 2,4,6 bank and gotten them backwards, then spend days trying to figure out what I did wrong, testing every wire at the computer, replaced a lot of sensors, pulled the timing chain...

What I'm thinking at this point is one of three things: Crank position sensor, ICM, or wireset being installed correctly (I believe you're ruled out this one).

[aj_94ssei]

I've already installed a new ICM but crank sensor could be a possibility still. Whats a good way to test the crank sensor?

[MattStrike]

Whats a good way to test the crank sensor?

An oscilloscope, but the cheapest you'd be able to find would be about $100. I've yet to find a good cheap way to check the crank sensor short of replacing it with a junkyard unit. I would start with the wiring to it, a much cheaper DMM will have a open/short circuit tester, that you can use to verify the wires between the crank sensor and ICM are not broke. Also check the connector for corrosion and the terminals to make sure the little tounge inside isn't bent/broken.

A timing light (also not cheap) with a reference mark on the balancer relative to #1 pistion TDC (0 degrees advance) between the balancer and the subframe can also tell you if you have proper spark timing, but again it takes about 25min to setup and mark 0,10,20 degrees and you have to buy a timing light ($80 or so).

[Arrowhead]

Call me crazy but, I always suspect the battery for many problems these days. They just don't make them very well anymore.(most, not all ) Good things sometimes happen when you put in a new,good battery.

[aj_94ssei]

well, tried to start it today to take it to a mechanic. Can't get it to stay running at all. looked underneath at the cat converter and was bright cherry red. I disconnected the exhaust in the front of the converter and it made no difference in how it ran. (start idle for a few seconds and then die) If I try to rev it up, it will hold a certain rpm for a few seconds and then it cuts out and dies. Any ideas on the cause of the cat converter? rich fuel mixture? lack of spark? thanks.

[MattStrike]

It is definitely running rich (misfiring or significant timing issue) for the converter to get red hot. So to understand, it will start, run for a few seconds, then cut out? Is the intake plugged? And it still starts hard, runs rough?

Might be a good idea to do a compression test still, but the crank sensor might still be the cause. Have you checked the wiring to the sensor?

[aj_94ssei]

Wiring to the crank sensor is good.. but not sure about the sensor itself. when it starts will idle rough for about 10 sec. then rpms go up and down 600 to 1300rpm then die. if I floor it for 1 sec. I can get it to keep running longer but I have to keep flooring it and letting off every few seconds to get it to stay running. If I try to hold a rpm it will cut out and die after about 2 seconds. Still starts hard and runs rough. Monitor fuel pressure remains over 40 psi. Whatever the problem is, it seems to be getting worse... since now it doesn't even get any better when the engine temperature starts to come up unlike before.

[1oldman]

well, tried to start it today to take it to a mechanic. Can't get it to stay running at all. looked underneath at the cat converter and was bright cherry red. I disconnected the exhaust in the front of the converter and it made no difference in how it ran. (start idle for a few seconds and then die) If I try to rev it up, it will hold a certain rpm for a few seconds and then it cuts out and dies. Any ideas on the cause of the cat converter? rich fuel mixture? lack of spark? thanks.

It is definitely running rich (misfiring or significant timing issue) for the converter to get red hot. So to understand, it will start, run for a few seconds, then cut out? Is the intake plugged? And it still starts hard, runs rough?

Might be a good idea to do a compression test still, but the crank sensor might still be the cause. Have you checked the wiring to the sensor?

Do you think the "cat" might be plugged? - BC

[aj_94ssei]

i disconnected it and there was no change.

[1oldman]

I would not suggest changing the PCM without the use of a Diagnostic Error Code Reader that could graph your different engine electronic devices (ie: TPS, IAC, MAFS, AIT, Crank Position Sensor, Cam Position Sensor, Engine Control Module, and Injectors). My experience has been good PCM's are very difficult to come by. I just got my ole '92 squared away. I thought it was the PCM. Turned out it was the Engine Control Module. I bought an A1 Cardone remanufactured ECM. Took two to get a good one. That is why it is so important to get a Diagnostic Error Code Reader that can graph your different engine electronic devices or an oscilloscope. Otherwise you'll just be throwing parts and $$$ at it. That gets expensive. JMHO - BC

[aj_94ssei]

Tested spark, only had spark on cylinders 3 and 6 and that was week. switched back to original ICM.. i have spark again but it still cranks hard like before. any idea what kind of problems could kill the icm? Thanks

[1oldman]

Tested spark, only had spark on cylinders 3 and 6 and that was week. switched back to original ICM.. i have spark again but it still cranks hard like before. any idea what kind of problems could kill the icm? Thanks

Typically, you should be getting error codes if the ICM is that bad. I have had the ICM fail and not give an error code(s). Check your wiring harness to the ICM and the connector on the wiring harness to make sure the connectors are clean and making contact. It is possible that both of your ICMs are defective. I have seen my son go through as many as four replacement parts before getting one that is working properly. I have gone through as many as three. - BC

[aj_94ssei]

what factors affect timing when the computer is in open loop or (engine cold)? Only the crank sensor?

[MattStrike]

There might be some advances in the way this happens, but if memory serves:

The coolant temp sensor in combination with the O2 sensor determine open/closed loop. The computer waits until sufficient activity on the O2 sensor is read until it switches over. The heated O2 sensors allow much faster switch to closed loop. Closed loop is for adjusting the Air Fuel Ratio, not so much for timing. It usually takes 2-5 minutes to switch over.

The ICM directly controls timing during the starting routine using the crank sensor to know when to fire. The computer monitors engine temp, ambient temp, mass airflow, and the cam sensor signal to adjust the air-fuel ratio and time the injectors to allow (reletively) immediate startup. The computer will override the ICM at about 450RPM - usually during the first few cylinder fires - and take direct control of the spark timing, it gets the crank sensor signal from the ICM. Full spark advance will not occur until closed loop (45-60 degrees depending on load, rpm, temps, etc...). The knock sensor tells the computer when it needs to retard the spark timing to prevent pre-detonation, but it is only a 'reactive' to detonation.

Have you done a visual inspection of the crank sensor? Check for the vanes on the harmonic balancer being damaged, or rubbing the sensor, check for sensor damage? Have you done a compression test yet?

[aj_94ssei]

I did a visual inspection of sensor and wiring all looked good. Have not done a comp. check yet.