I have an 86 coupe with the original ECM. I'm having several problems which lead me to believe I'm having ECM trouble. The main fan is on all the time. engine runs good until it warms up, worse with the heater on. After it is warm, the engine sometimes seeks when it is trying to idle, varing from 300 to 800 rpm.

Woud it be better to get a replacement ECM or upgrade to the later ECM (89 I think) and repin the connector?

What are the advantages of upgrading the ECM?

 

First off, I'm no expert on this and there are a lot of other folks on here that know much more than I about these EFI control systems.

But with that said, here's what you may want to consider.

Your ECM is more than adequate to control your engine in a pretty wide range of temperatures, altitudes, fuel differences, etc.

It would be better to find the problem you have BEFORE you do any major modifications to the control system. If not then you will have THAT problem PLUS al the variables you threw into the "mixture" (no pun intended) by changing out the control system.
There's a LOT of information on here if you're willing to find it and read it.
Here's a couple of tips to help fix your current problem.

1) Clean your throttle body.
2) Clean and adjust your IAC.
3) Check Your Fuel Pressure and make sure it's within specs.
4) Install a new rotor and Distributor Cap
5) Install new plugs
6) Install new plug wires if yours are a few years old.
almost forgot....
7) Change the fuel filter and get a load of good fuel (93' Octane)

Do just one thing at a time and see if the symptoms change (better/worse)
If the problem is still there after doing these things get back on here and report what you found and request more help.

Good luck with it.





[Modified by VetNutJim, 11:07 PM 4/15/2002]

 

'86 through '89 use the same ECM, referred to as the 165 (last three digits of part #) The next upgrade is to the speed-density ECM used in '90 and '91. In Corvettes, this ECM is called the 727, is located under the hood, uses completely different connectors and pins, and would be an enormous undertaking to install. The F-body version is the 730 ECM, and can be retrofitted with repinning and a little rewiring. I just finished my 730 install this past weekend.
All that said, unless you are making major mods (like a 383 or 406), there is no reason to change to the 730. The 165 works pretty good.
I recommend you get some diagnostic software from Craig Moates, TTS Datamaster, or Diacom, so you can troubleshoot what is wrong with your system. Something is broken, and you will need to fix it before you start modifying.

Good Luck

 

I've done all that you listed. I have hooked a diacom on and not found any problems. I may borrow my friend's diacom and monitor the car while driving to see if it is any different. we only checked it in his driveway, that was about 3 months ago. Lately I've noticed when I run the A/C I'm losing alot of power. I will put a new fuel filter on this weekend. I don't see how any of the things listed would affect the fan running constantly or the car running worse with the heater on.


[Modified by fiddleplayer, 7:39 AM 4/17/2002]

 

The symptons of the fan running continuosly and the engine not running correctly are most likely not related.
In other words you have one malfunction that is causing the cooling fan to run constantly.
You have another problem with the engine not running correctly.
The only way to fix this stuff is "divide and conquer".

Fix ONE thing at a time. Isolate each problem from the others.
Having multiple troubles is not uncommon. Particularly when a malfunction doesn't totally disable the vehicle.
You know how it is when something goes wrong,if the car will still run, we procrastinate until something else goes wrong, then something else.
So when you do finally get around to solving ONE of the problems there are actually several things to fix.

Now back to your car. You've got a couple of choices to make and only YOU can decide what's best for you.
First you must decide if you have the skill and more importantly the PATIENCE to do your own troubleshooting.
Some of this stuff can get really involved.
Unfortunately, NO ONE can take a simple description of ONE problem and hand you a solution "on a silver platter" to ALL your malfunctions. That's just NOT going to happen in 99.999% of the cases.
So if YOU don't have the patience to do this tedious stuff the best thing for you to do is take your car to a good mechanic and pay him to do fix it for you.

If you REALLY want ot do it yourself BUT you need very personal assistance and you want it get it from this forum, YOU will have to do things to give the folks here the insight to solve your problem for you.
After all, if you KNEW how to fix it you wouldn't be asking for help HERE.

If you want to jump in and do a complete redesign of your engine control system then more power to you. I surmise since you are having difficulty getting the one that REAL engineers designed and built to work properly that perhaps that may be a little more of an endeavor than you should attempt right now.
But it's YOUR car, do what you want.

If you want help from the forum members you will have to do the test necessary for someone to have SOME input as to what may or may not be wrong.
Believe me when I say there are a LOT of folks on here that could diagnose your problems and reach a logical solution in a few minutes if they had "first hand" access to your vehicle.
Look at this way: Anybody that tries to help you here is doing so using their OWN TIME and EXPERIENCE to try and help you.
Also they are EXTREMELY HANDICAPPED from the standpoint of they can't hear the engine run, can't touch it, feel it or smell it.
Therefore YOU have to be the eyes and ears.
COMMUNICATION is CRITICAL.
If you had said I have checked this and this and this and I have replaced this and this and this and my engine is doing this at this temperature then someone could maybe skip a few of the preliminary steps necessary to reach a conclusion as to what the problem is.
A Diacom data pull 3 months ago is not going to cut it.
For anybody to help you here, they will need to know what you did yesterday and today to find the problem.
You will also need to give REAL data such as:

Idle RPM:
Fuel pressure at Idle: 35 PSI
Fule Pressure at 3000 RPM: 12 PSI
Plugs indicate running Rich/Lean

And on and on, until all the problems are solved one at a time.
No one can diagnose from generalities. Input must be specific numbers.

So it's up to you. Make up your mind whether or not you want to receive help here and fix it yourself or take it to a mechanic and pay him well because he knows how to fix it AND he'll be able to diagnose it "first hand"..

Good luck with it.


[Modified by VetNutJim, 8:39 AM 4/17/2002]

 

If the ECM is stuck in diagnostic mode or limp home, then the fan will be on all the time. Most but not all problems that cause limp home also set a code, but you have no codes set. :confused: Also possible the relay is stuck or temp sensor is bad or a wire is shorted, or ECM relay output is stuck on, yada, yada ...

I would first check the temp sensor because it sounds like the ECM is not tracking engine temp very well - inappropriate fans etc. I believe the ECM has its own temp sensor separate from the dash display.

I would also replace that ECM just because the original design has experienced a high failure rate. Your original 1227165 ECM was redesigned to address the problems, the new number is 16198259, but if you order a reman 165 nowadays you do get the redesigned unit. Also the PROM has been updated if you have Al heads - TSB 86-162 describes the fixes. If you have iron heads and MT I know for a fact that the original PROM is exactly the same as today's replacement, so a new PROM is not always necessary when replacing the ECM.

 

Okay, first for a little background. It's an early 86, around the 750th made in that year model. Yes, iron heads. Yes, the fan switch (replaced) is up by the thermostat and the dash gauge temp sensor (replaced) is between No. 8 & 6 cylinders. The check engine light has not worked on the car since I got it two years ago. The car has never ran in what I would call peak performance. I'm down to around 15 miles to the gallon. I have been chasing gremlins in that two years. I've had it on the diacom a few times with numerous ideas, changed several of the sensors due to age. The last time it was on the diacom it had no codes and was doing the same thing it is now. Jim, I've been an electrical engineer in the defense industry for 27 years and work on systems more complex than anything in the vette, daily. What leads me to believe that there is an ECM problem is the car will run worse when the hot air from the heater heats up the ECM. The current ECM has spent 16 years in a corvette, not a cadillac, so there could be some cracked solder joints, corroded connector contacts, or maybe even a bad component.

My original question was would it be better to upgrade the ECM or get another just like the one that is there now. I've read on some posts that the baud rate is faster in the later models and was wondering if it was worth updating or not, since I will be buying one anyway. I will eventually find the problem, but I have more things to do that just work on the vette. And while I have the ECM out, I'll be able to determine why the freakin' check engine light does not work.

So, here's what I'm getting from these posts.

A refurbished ECM is better than my original and probably adequate for my application. If I am going up upgrade to a 383 or better, the later model ECM is suggested.

Anyone haver any suggestions as to where I might purchase a quality replacement ECM?

 

Hey, fiddleplayer. I have the same idle seek problem - 400 - 800 rpm. I don't have the other problems. Let us know if the ECM change helps anything.

Good luck.

 

Run over to eBay and look for an ECM like the one you have.
I've bought several off there and knock on wood they all worked.
THis will hopefully give you a baseline to work from.
You'll should be able to get one for about thirty bux.

If the car runs better then you've identified and verified the problem.
If you want to look for bad solder joints, there's not a lot to see on the PCB of these ECMs so that's a good test you can make with nothing more than
a good magnifying glass.

Examine where the connectors come into the PCB and see if there's any bad solder joints possibly from heating and cooling all these years.

Have you shorted pin A&B (I think) and read the codes off the computer?
If the MIL (malfunction indicator light) doesn't work when you do this then the ECM is definitely bad OR the bulb is bad.
If it's ECM related it looks like some functions are working and others not working.

There's a multitude of things that COULD cause these symptoms (as indicated by the last post).

Is the timing ok? Has something gone wrong that may have affected the timing?
What do the plugs look like? Running rich? Lean?
I know it's a peeser to get them all out but they are good "forensic evidence".

Let us know what you find out on it. Give us a few more hints if you can.

Best regards,

 

It's supposed to rain here all weekend. I'm going to try to borrow the diacom again and spend some time routing out this problem. Thanks for the EBAY suggestion. I will start watching that. I've noticed in several other posts that alot of members have spare ECM's. I'm not looking forward to accessing the computer. When I redid my interior, I couldn't get the passenger side cover under the dash off, so I just applied the carpet with it in place. I guess it will have to come off now. I'll let you know what I find.

VetNutJim, I sorry if I offended you earlier. I wasn't really asking for help troubleshooting. I was asking, since I'm probably going to replace the ECM would I be better off going with a later model ECM. I guess I just didn't make that clear. Once again I applogize. Thanks for the tips and I will use your suggestions as a check list.

 

Hi FiddlePlayer,

No problem whatsoever.

Send in what you find and we'll conjure up some more stuff to check.
I have replaced an expensive part so many times to only find out it was some obscure ten cent thing that was causing the problems.
But a low cost replacement ECM won't hurt much even if that's not the problem.
Sure wouldn't want to see you pay retail for one of those things.
This way if the ECM is NOT causing the problem, you've got a spare anyway.
Please keep us posted so we can learn too.

Best regards,
Jim

 

the baud rate is faster in the later models

'86 - '89 all communicate at 8192 baud. The '86 can also communicate at 160 baud (through the magic of software), but why bother since it's just there for backward compatibility. Hardware is identical for all these years.

the car will run worse when the hot air from the heater heats up the ECM

Yeah, I remember chilling (Freon spray?) each transistor in one channel of an amp to find out which one had become a white noise generator, or was it pink? Heat sensitivity is definately a tip off. An alternate explanation is that the power filtering on the ECM board has degraded (dried electrolyte, etc) and the fan motor, which is electrically noisy anyway, is causing transients on the power rail. Either way, ECM could stand replacement.

Anyone haver any suggestions as to where I might purchase a quality replacement ECM?

I got a used one for $40 (here at the C4 parts for sale forum) to keep as a spare but it had some problems so I used it as the $90 core charge at Autozone. The ECM price was less than the core charge, $85 - go figure. Anyway, now I have a known good spare in hand, which for me was a prerequisite to beginning EPROM burning.

iron heads

Then you probably don't need to by a new memcal - EPROM contents are unchanged.

If I am going up upgrade to a 383 or better, the later model ECM is suggested

This may be a minor distinction, but the upgrade is to a MAP based system which allows accurate fuel control at higher air flow than a MAF based system. The later model ECM itself is still 68xx Motorola technology, not much of an upgrade, it just has specialized hardware on the board designed for speed density control. Jump right in if I am wrong.

the fan switch (replaced) is up by the thermostat and the dash gauge temp sensor (replaced) is between No. 8 & 6 cylinders

Here is where my memory fails and I don't have the service manual nearby. I recall the dash sender betweem 6 & 8. There is a switch in the head between 1 & 3 to control the aux fan if so equipped. But isn't there also another sensor (not a switch) somewhere in the front of the engine that directly inputs to the ECM, that is used in fueling, main fan turn on, etc?

 

Well, I borrowed the Diacom today and pulled the codes. 14,36,42,43. I'll chase these down and see where it leads me. Opened a new thread in tech.
If I get stumped, I'll be back.