MEMCAL replacement... should I?

HallenTi · 05-23-05, 01:34 PM

I have had several problems with my 1990 coupe over the years, and am wondering if replacing the MEMCAL might be the answer.

The one problem that I am sure could be corrected by the replacement is the knock sensor. The Helm Chevrolet repair manual I have points to the MEMCAL as the culprit when I do the diagnostics for it. I have the proper voltage at the wire at the knock sensor, and the proper resistance range when checking the knock sensor itself. The diagnostic says if this is the case I have a faulty MEMCAL. I would like the knock sensor system to work properly, so I may set the timing properly and have the system function as designed. I have also done the diagnositc where you rap on the engine while it runs and watch the timing, and there is no change in the timing.

Another problem I have is cold-drivability. It "hesitates" somewhat when cold, requiring some "babying" until warm. I have tested the MAP sensor voltage at ambient and under vacuum, TPS voltage at closed (.54v) and WOT positions (5v), both the MAT and the coolant temp at 32 degrees and 212 degrees, and all appear to be withing the specs. When warm, this hesitation doesn't exist. I have also replaced the O2 sensor, however, I know that when runing cold, the ECM does not take the O2 reading into account, but it had many miles on it and was an inexpensive change. Are there other sensors I have missed that are used during cold operation?

The wires, cap and rotor are new, and the wires properly routed and clean. The plugs are also new (Bosch platinum) and gapped to specs. I have used techron fuel injector cleaner recently as well - double dose. I can't find anything (at least on my own given my limited diagnostic capabilities), and not certain that with the ODB I system the 1990 has, whether a shop can "read" the various sensors with any more success than I can with a DVM as they more eaily could on an OBD II system in later years). Those that have had similar problems, or that have had success or failure from either the Chevrolet dealer or independant shops with "diagnostioc testing" I'd like to hear from you. I wouldn't mind spending some money for a thorough diagnosis if available. Thanks in advance, Tim

HallenTi · 05-29-05, 10:54 AM

Ttt ^^^

WhalePirot · 05-29-05, 01:28 PM

Did you try reseting the ECM? Like any computer, a reboot can do wonders. Certainly, the price is right. It may have learned some bad settings which, when erased may improve cold driveability, which sounds like insufficient fuel.

Good show on the troubleshooting and posting!

HallenTi · 05-29-05, 04:46 PM

Yes, the "reboot" is one thing I have learned over the years does sometimes work wonders!!! This time, it hasn't helped however. I downloaded the WINALDL program, and am getting parts to assemble a data cable to my laptop... then I can at least (from the looks of things) see the various pieces of data all at once, and get the readings from more sensors than I have time (and patience) for. I will see what thar reveals.

mdsbch · 05-29-05, 07:12 PM

Please let us know what you find out. I have some of the same symptoms with my 85 and have replaced al of the same parts you have, including MAF and EGR valve. I get intermittent codes for the EGR, MAT, and now one that says "memcal missing". They all seem to disapear after the car is shut off and restarted. They seem to return normally about 10 to 15 minutes after starting it up after it has cooled down and normally when at speeds over 65 MPH. Do you have any of these other symptoms with yours.

Mike

HallenTi · 05-31-05, 11:18 AM

I have never received the "MEMCAL missing" error code, but often get a code 42 (if I am remembering the number corectly) which points to the electronic module in the distributor. At one point I replaced the module, then it later failed dead (left me on a freeway on-ramp) and had the same code, so I replaced the distributor with a rebuilt unit as the pole piece and the rest of the innards had almost 150,000 miles on them. Sometimes I still get the code, but it hasn't left me stranded, and a reset clears it usually for a period of time. I will continue to check things (have parts to built a data cable for the ALDL to laptop and the WINALDL program. Hope that helps me check/diagnose the sensors that I haven't checked yet.

As far as the MEMCAL goes, I have been told that as long as the "service number" is the same I could get one anywhere (have seen them on eBay for a reasonable price) but not sure which numbers are the service numbers. Will have to look into that a bit more.