Possible issues with sensors/DME...need help

gregbus · gregbus's Garage

I have an 86 w/ stock 3.2 and B&B muffler. Car was running great, but was driving home the other day and the car just died. Turned over fine...just didn't want to start. We swapped the fuel pump, DME relay. No go. Then swapped out the DME computer from a car that was running fine (another 86 3.2 motor w/ chip). Started and ran rough, but thought it may have just been the chip.
Then...back to the same original problem after trying to start it again. Wouldn't start.

We swapped out the two sensors (Cylinder Head and RPM) from the other motor. Also looked at the flywheel sensor and it looked like it was in pretty good shape. The car started but ran rough. But back in the original DME computer and started it up. Still runs pretty rough and quite rich. Backfires a little and idles terrible.

Any thoughts or ideas would be much appreciated!!!

cnielsen · 09-16-2008, 08:25 PM

I think Bernie said the car was running lean.

Anyway, parts ruled out are the DME, DME relay, fuel pump, and the two sensors (RPM and TDC).

It almost sounds like two different problems...one caused by the other.

Now the car starts every time but runs poorly, takes throttle but spits and pops, smells like spent fireworks.

Could the initial problem of a no-start and several attempts to restart cause another problem?

We are about the swap out the Air meter control unit but are open to ideas after that.

db_cooper · db_cooper's Garage

Could be the temp head sensor? Don't have the specs handy, easy enough to check with an Ohm meter. The "fireworks" odor could be a rich condition and the catalytic converter being overworked, if the sensor is open. Might be a loose connection to the sensor too. Worth a look.

cnielsen · 09-17-2008, 06:25 AM

We checked that with an ohm meter and it appeared to be working...a little higher than spec but not too far off. Anyway, I think that is the first thing to look at...unplug it while the car is running and see if it runs any worse, if not, that may be the culprit.

Thanks for the idea!

rick-l · 09-17-2008, 06:48 AM

can you put the questionable DME in the car that runs fine?

jbrinkley · 09-17-2008, 06:52 AM

money on AFM

cnielsen · 09-17-2008, 06:59 AM

I have not placed the suspect DME in the good running car yet as they are in different locations. I did confirm that the second DME continues to run fine as I reinstalled it in the good running car last night.

The CHT sensor and the AFM are the next step. Actually have a spare AFM that I can hook up to test. However, the question there is why would this instantly cause a stall, then a non-start, then a start but runs poorly issue. Seems like two separate issues that happened to arrise at the same time.

Hmmmm, carbs are sounding good about now!

jbrinkley · 09-17-2008, 07:09 AM

time between switching or swapping parts could have an effect on the cht operation.
Could both cht's be bad?
A car could start and run for a while with a failing cht. And then fail to warm start.

cnielsen · 09-17-2008, 07:14 AM

Could it cause a stall though at freeway speed? BTW, the car had been driven about 15 minutes when it stalled on the freeway.

jbrinkley · 09-17-2008, 07:15 AM

yes it can, I've seen it happen. 15 minutes is probably about perfect time time to, depending on ambient temp.

but I'd swap afm's before I jacked up the car. Is there a cat?

ischmitz · ischmitz's Garage

Good call on swapping the suspect DME into a good running car to rule it out.

Next, a systematic check of sensors and actuators, etc:
- CHT is only suspect when running rought during cold engine. At temperature it should not have much of an effect
- O2 sensor
- AFM
- fuel pressure regulator
- leaking or stuck/clogged injector
- bad/stuck ICV

Cheers,
Ingo

cnielsen · 09-17-2008, 07:23 AM

yep, original cat right now. Cat bypass is on order and supposed to be here on Thursday. New B&B muffler recently installed too.

jbrinkley · 09-17-2008, 07:43 AM

ischmitz knows plenty more than I do. But I experienced the opposite. Cold start fine, warm or hot start = fail.

ischmitz · ischmitz's Garage

Quote:

Originally Posted by jbrinkley

ischmitz knows plenty more than I do. But I experienced the opposite. Cold start fine, warm or hot start = fail.

no no - I wouldn't say that. My observations of the CHT are from a 3.6 where it doesn't harm a warm engine. The DME simply ignores a missing or open CHT and assumes the engine to be at operating temperature. On the 3.2 what you have seen may well be true for the 3.2. So I would check the CHT. Measure resistance at room temperature to be around 2.2 - 2.8k Ohm.

Cheers,
Ingo

cnielsen · 09-20-2008, 04:26 PM

OK, so a little update...

Got the cat bypass hooked up...wanted to do this anyway, but it made no difference in the poor running condition.

Swapped out the AFM from a known running car and it also made no difference.

Then, started the car and unplugged the CHT sensor while it was running. This made no difference, the car still hunted for idle and backfired. Soooo.... from deductive reasoning, it sounds like the CHT sensor is not doing what it is supposed to do.

A new CHT sensor is on order and hopefully that will put it back on the road!

Thanks for all your help and suggestions, it ultimately led us in the right direction.

ChrisBennet · ChrisBennet's Garage

I would recommend against putting someone elses DME in the car being tested. A car can hurt a DME but a DME can't hurt a car. Always test your DME in a known good car - don't try/fry someone else's DME in your own car.

The cylinder head temperature (CHT) sensor on a 3.2 can certainly bring your car to a stop as it can make the engine go full rich. It will feel like it's bucking before then.

The CHT reads around 3600 ohms cold and 300 hot. A bad CHT or a bad ground with the earlier single wire (1 pin in the connector) looks like an open circuit (Infinite ohms which the computer reads as super super cold.) To compensate it makes the mixture super super rich. A meter won't always tell you if they are bad as they can be intermittent. I unplug the CHT and stick a 1/4 what resistor into the connector - say 3.3K ohms for a cold motor and 330 for a warm motor and take the CHT out of the equation that way.

-Chris

ischmitz · ischmitz's Garage

Quote:

Originally Posted by ChrisBennet

...but a DME can't hurt a car

Not quite right. A short in the injector final driver stage can damage the injectors in a good car. Once the peak-and hold current limiter is not working the low-impedance injectors can overheat and burn out. As a minimum you will flood the engine. Ask me how I know. Let's just say fuel was dripping out of the tail pipe....

On the other hand I could see that a bad ICV (or short in the harness) can damage a good DME. So there is always risk with swapping DMEs either way without doing some basic testing first.

Ingo

Wayne 962 · Wayne 962's Garage

Ingo, in your experience, how often is it a problem with the DME? I think that about 9 out of ten times, the problem lies in components or the harnesses, not the actual DME unit?

-Wayne

ischmitz · ischmitz's Garage

Wayne,

I agree that most times it is something else other than the DME. The DME is amazingly bullet-proof and it takes quite some abuse to damage it. Most times in loose order of priority I see issues with:

1. The DME relais
2. The flywheel sensors or CHT sensor
3. The alarm unit (if fitted)
4. Stuck injectors/fuel filter, fuel pressure, fuel pump
5. The AFM
6. The ignition switch

I think one day I have to get off my couch and write up a little troubleshooting article about no-start conditions on the 3.2 - something like a step-by-step diagnostics with simple tests.

That reminds me, when is the Pelican Wiki coming online?

Ingo

burgermeister · burgermeister's Garage

If the CHT gets loused up, isn't the limp home fix to ground it (as opposed to leaving it open)? I thought grounding the CHT wire should make it run OK if the sensor is bad. My memory sucks, so don't take my word for it ...