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Bad PCM?


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#1 rich_e777

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Posted 03 August 2017 - 05:41 PM

So I`m stumped on this one, 1994 3100 CS coupe. Cranks, but wont run unless the gas pedal is depressed but its rough then dies when let off. Was driving around fine one day and stopped at a store then it all started. Replaced 3x sensor harness and confirmed spark at 1-4 coil and fuel pressure at shrader valve. The only thing I can think of is the fuel injectors not spitting out gas or getting the command. Any ideas?



#2 Imp558

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Posted 03 August 2017 - 06:13 PM

Vacuum leak or a ground. Bad PCMs happen but not frequently. I just replaced the BCM in her MC only to find one of the ground bolts under the hood was stripped and making a poor ground from body to battery.
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#3 Schurkey

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Posted 03 August 2017 - 06:52 PM

So I`m stumped on this one, 1994 3100 CS coupe. Cranks, but wont run unless the gas pedal is depressed but its rough then dies when let off. Was driving around fine one day and stopped at a store then it all started. Replaced 3x sensor harness and confirmed spark at 1-4 coil and fuel pressure at shrader valve. The only thing I can think of is the fuel injectors not spitting out gas or getting the command. Any ideas?

Schurkey's Perpetual Advice:  The answer is as close as the nearest scan tool.  The data stream rarely lies.

 

You can make guesses, replace parts, visually inspect all sorts of items including the wire harness.  Sometimes guys get lucky.

 

Might be stuck EGR.  Might be faulty coolant sensor.  Could be defective TPS.  Could be a vacuum leak.  Could be this, could be that.  It does run when the gas pedal is depressed.  Therefore it probably has fuel, spark, compression, and adequate exhaust.  Doesn't mean that those items are blameless, just that it's somewhat unlikely that they're at fault.  If you were to verify compression pressure in ALL SIX, and it's good, that also verifies cam timing.  A running compression test could verify cam lobes, lifters, pushrods, rocker arms.  You did verify fuel pressure, you've gone a long way to assuring that the fuel pump and fuel filter are OK, and that the injectors are receiving adequate input pressure.  You crammed a spark-tester on each of the plug wires , you've verified the coils, plug wires, primary wiring, some sensors, etc.

 

Put a scan tool on it, and in fifteen minutes you'll learn more than a day's worth of dicking around.  Even if the engine doesn't run, you can test the intake air temp sensor, coolant sensor, MAP sensor, TPS, knock sensor (have a helper rap on the engine with a brass hammer), crank sensor (crank engine, look for RPM on the scan tool) and others in minutes not hours.


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#4 rich_e777

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Posted 03 August 2017 - 09:45 PM

Really cannot argue that logic, I`ll bite the bullet and get a real scan tool tomorrow. I was trying to solve it on my own but someone called codes on me for having it and other cars being worked on in my driveway so I need to get it started and at least turned around so it appears to be functional.



#5 Galaxie500XL

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Posted 03 August 2017 - 09:52 PM

Going to be hard to find an OBD1 scantool these days new...


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#6 rich_e777

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Posted 03 August 2017 - 10:32 PM

I ordered one that does OBD 1 and 2 for about $150 off ebay. Equuis brand IIRC. If it doesnt work then I`ll track down an actual Tech1 or MT2500 several have mentioned using before with good results. I want one I can use with the FSMs.



#7 Galaxie500XL

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Posted 03 August 2017 - 11:56 PM

If you lived closer, I'd have let you borrow my OTC 4000E, or my Actron CP-9110, or my laptop with TunerPro, and a Saar built ALDL cable.  They work great on our cars...I even have the cable and cartridge for the Actron for the LeBaron.

 

I seem to be collecting scantools.... :think:


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#8 Grandprix1

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Posted 04 August 2017 - 12:45 AM

X2 on the vacuum leak. My GP did the same when I forgot to put the vacuum line back on the TB. no fuel pressure regulation. Wouldn't start unless pedal was pushed and would rev to 3k then stall out Sent from my iPhone using Tapatalk

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#9 rich_e777

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Posted 26 August 2017 - 08:42 PM

So after a huge headache dealing with ebay I still have no actual scan tool. It was an OBD2 scan tool with OBD1 code reading capabilities, not both but got my money back.

 

I found this http://www.ebay.com/...CkAAOSw0hlZN5BC Seems to have everything but want to make sure before I get this one.


Edited by rich_e777, 26 August 2017 - 08:43 PM.


#10 RobertISaar

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Posted 26 August 2017 - 09:07 PM

if you NEED those kinds of capabilities, that might be a  decent  investment, otherwise ALDL cables are cheap enough to buy(and WAY cheaper to build).


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#11 Imp558

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Posted 26 August 2017 - 09:22 PM

Yeah a cable to the laptop would be cheaper. the Snap-on brick was a really awesome scan tool at one time, considering you can get it for 249 dollars right now that's not a bad price. I paid 300 something for mine last year. when you're looking at the mt 2500 you need to make sure it has all of the keys and cartridges you need. that one does not have the programmable cartridges in it for GM which means that those dedicated cartridges that are with it have to be able to cover the range of years that you're looking at. Ideally you want one that has the cartridges that are programmable and the early ones. programmable cartridges will run much newer in model years but the early one will contain more troubleshooting information and capability for the earlier models so it is useful to have both

Edited by Imp558, 26 August 2017 - 09:23 PM.

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#12 rich_e777

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Posted 27 August 2017 - 04:49 PM

if you NEED those kinds of capabilities, that might be a  decent  investment, otherwise ALDL cables are cheap enough to buy(and WAY cheaper to build).

I do have that aldl cable i think i bought from you and have a TP folder on my laptop but I could never get it to connect to the car. Its been some time now but iirc there was an issue regarding a 4 letter code on the pcm not working with any .ads or .xsf files. I`ll read through the guides and see if I can pinpoint the issue later today. 



#13 RobertISaar

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Posted 27 August 2017 - 05:50 PM

let me know if you can't get it going on your own, I do have a little time today to explain/correct things if needed.


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#14 rich_e777

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Posted 17 September 2017 - 05:30 PM

So Ive been tinkering with this on and off since last posting on this thread and finally found the right rabbit hole. EGR valve was stuck open. The only trouble code I`ve ever seen on this car besides leaving the air temp sensor unplugged, was for the EGR valve literally 10 years ago. So time for a new one this time. Ive cleaned the old one so many times and the central gasket is worn out and cannot be found so i had used some high temp gasket maker in its stead which seemed to work up to the point it didnt.


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#15 rich_e777

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Posted 17 September 2017 - 11:16 PM

Looking like it might be a combination off things, still including a bad EGR valve but the only way the car runs with the one from the vert is without the gasket. I`m going to pull the plenum and check for a good mating surface and should have the new valve come in tomorrow. When it did first start up though it did sound exactly like a time forgot to torque a rocker arm to spec which is a bit unnerving, could be from the engine just sitting for several months.



#16 Galaxie500XL

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Posted 18 September 2017 - 03:39 AM

I'm pretty sure I gave you a spare EGR when you bought the convertible....
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#17 rich_e777

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Posted 18 September 2017 - 01:28 PM

Not an EGR valve, was an airflow sensor, an extra front wheel hub, i think a drivers side A pillar seal and the original PS pump, and an air box. And some tubing made from unobtainium for the LQ1. 


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#18 rich_e777

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Posted 18 September 2017 - 09:23 PM

Ok so the EGR swapping must have been some sort of fluke but at least the new one doesnt leak from the gasket. Everything I test points to a fuel problem yet the fuel pressure and pump voltage tests ok, so might be a good time to re-look at that passkey resistor I had spliced in vs the resistance value in the keys I have, sure enough something is up with the resistor. The pellets in my keys get a solid 3.0 at 20k= 3000ohms? (I used to know the conversion better) while testing the spliced in resistor it gets 2.3 at the same setting.



#19 Imp558

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Posted 18 September 2017 - 11:24 PM

It runs, it's not VATS
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#20 Nas Escobar

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Posted 19 September 2017 - 08:01 AM

Yeah VATS is out of the picture unless it won't start at all. If signs are pointing to a fuel issue... Well Schurkey already mentioned the bucket list of could bes... I went thru a similar issue with an LH0 and a lot was fried. To make a long story short, get an ALDL cable and download tuner pro on your laptop. Then do a run and record a log. You need to basically look at your BLM and INT values to see what's going on. Knowing how much KR the engine is getting will help too.

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