I've been trying to trouble shoot a driveability problem, that doesn't seem to have any problem codes, although it is hard to tell as the SES light seems to flicker sometimes, when in the diagnostic mode. I get the 12 code but not always in sets of three. I took it to an OReillys Auto Parts to get a code check, they said no codes. In an effort to narrow down the problem, I went to section "B" symptoms. It sounds like the "Rough, Unstable, Or Incorrect Idle, Stalling", Symptom, as described in Service Manual section 6E3-B-7. When it is warmed up it runs smoothly when above idle speed. WOT smooth, no hesitation, No missing. The SES light is on most of the time when driving. I have replaced the PCV and have used a Mightyvac to find and repair several hoses in the evaporative fuel system area. The ten year old gas has been run out and replaced with 93 octane and some Sea Foam gasoline additive. I started with the EGR system, no particular reason, it just seemed to be more likely than some of the other systems. I went through Chart C-7B section 6E3-C7-5. This lead me to grey wire 435 from the egr solenoid to terminal A4 of the ECM. If I followed the chart correctly I need a new ECM. If I understand the operation correctly the normally open EGR solenoid is not not receiving a ground from the ECM to modulate the EGR valve, so it is either fully open all the time???, or fully closed all the time??? I'm not sure which, but do you think this would cause the problem I have described or do I have additional problems? If I have analyzed the problem correctly, who are the preferred suppliers for ECM's. I understand that most are remanufactured units. Any other tips on ECM purchase and replacement. Thanks again for the C-4 training!

 

The EGR system is only activated when you're cruising down the highway, so you won't see it operating at idle. It's possible that the EGR valve is stuck open (it should normally be closed). That will cause a rough idle.

The ECM turns on the SES light when it sees a problem. The light connects directly to the ECM and nothing else. You should see error codes if the SES light is on. It's possible you've got a wiring problem (intermittent short to ground, since you said it flickers) in the SES circuit.

In my opinion your best option is to borrow or buy another ECM and try it. There are no more new ones left, so they're all rebuilt units.

The 1227165 ECM was used in a variety of cars (mostly Camaros and Firebirds) so they're relatively available. I bought one from an eBay vendor for $100. I have searched through the local Pick Your Part junkyard several times and have not found any there.

If you send your ECM to a rebuild facility MAKE SURE YOU KEEP YOUR MEMCAL. They are like gold $$$$$. They are typically removed and the rebuilt ECM will not have one. If you acquire another ECM for testing, swap your MEMCAL into it.

 

Cliff, Thanks. Just to do a recheck of the EGR I watched it with a mirror while I pulled 27hg, it held steady and when released I could hear and see the diaphram moving back into position. I rechecked the coil resistance of the EGR solenoid, 26.5 ohms. I ordered the 1227165 remanufactured ECM and pulled the MEMCAL, as you suggested. Wow only $60 bucks a pleasant surprise. The ECM that I pulled out of the car had a REMANUFACTURED BY GENERAL MOTORS CORP. SERVICE NUMBER 1227165 Sticker on the case. I've known the PO of this vehicle since before he purchase it in May 1988, so it must have had the ECM replaced during the first two years of its life. I'm still having trouble believing that this is the only problem with the poor idle, stalling, etc. I'll pick up the ECM tomorrow AM, will let you know if the problem is solved.

 

ECM Upgrade - The #1227165 ECM in your car is the original style L98 ECM used in the 86 to 89 Corvettes. Chevrolet has had a real problem with the internal circuit board cracking on these original units and when this happens there can be all kinds of problems including code 33, 36, 34, 15, 51 and a few more.

The repair service number of the unit you mentioned is an aftermarket rebuild, most likely painted black. I would not trust this old style ECM at all as I have found that just about all aftermarket replacement ECM units are of the old style and prone to failure.

The GM number you need to go back in your 86 C4 is #16198259. #16198259 is the GM replacement for the original #1227165 and
is a much better chip.

FWIW,

P.

 

Thanks for your input. After reading your post I called Advanced Auto Parts and asked about the two part numbers mentioned above, they assured me that the later part number/revision would be the one I would be getting and it would have a one year warranty. The rebuilt ECM and the one that I returned both had an unpainted aluminum case.
I installed the unit with the original MEMCAL. I did not notice a major difference in the idle, and did not get any trouble codes. However the SES light was much more consistent and easier to read the code 12"s. There was no more flickering of the SES light. Reading more posts lead me to the possibility that the idle screw had been adjusted, a call to the PO confirmed that it had been adjusted. So my next step is to have a crack at getting the idle screw back in spec. Under KBase C4 related Tech Tips I found an early C4 TPS & idle speed adjustment procedure by Lars Grimsrud. Is this the most current and accurate way to make these adjustments on my 86, or are there better procedures to follow? If so I will proceed with his procedure.

 

Have you checked base timing for 6 degrees with the esc bypass wire disconnected? Is the motor stock, or has the cam been changed?

 

I am almost positive that engine is stock, no cam change. ECS bypass wire?? are you talking about the "a to b" paper clip on the under dash OBD connector? I will have to borrow a timing light I only brought limited number of tools on my "up North for the summer" trip.

 

MrWillys meant EST (Electronic Spark Timing) connector. It's located near the windshield wiper motor and has a tan wire with a black stripe. Opening that connector disconnects the ECM advance so you're only measuring the mechanical timing (some call it the "base timing") of the distributor by itself.

ESC is Electronic Spark Control, which is what GM calls the knock sensor circuit.

 

It's actually the IGM module bypass wire if you need to be **** about it. Pin D5 on the ECM.
http://www.chevythunder.com/198689_f...dy_1227165.htm

 

Thanks for the clarification on the EST wiring. Are you leaning towards checking the base timing before doing the TPS and idle adjustment? If TPS & idle is the KBase/Grimsrud procedure the one you would use?

 

Here is a write-up on how to set your ignition timing. Also, you will need a throttle position sensor test tool to facilitate the proper setting of your throttle position sensor.

http://www.corvetteamerica.com/cf/di...ition%20sensor

 

How about filling out your user profile so we can help you without trying to figure out your profile on our own....

 

OK, I just added to my profile.

 

Okay Now you look like an official Corvette Forum member

 

Thanks!