1987 wiring and computer issues
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1987 wiring and computer issues
I have a 1987 Corvette. It will start, but it is a very hard start and runs rough. I took it to a shop. They replaced the intake (alternator, battery, “fixed” and exhaust leak). It does not have any smog stuff on it. It has the same issues it had before I took it to them.
I think it is mechanically good, it should run, but it does not
I think it is a computer issue or a wiring issue. It has a “known good” ECM.
Does anybody know 1) Is there a new partial wiring harness that could be purchased and swapped in? 2) Is there a way to check a computer (ECM)? I feel there must be a shop with some sort of electronic tester or monitoring device.
Any other ideas? It does not seem mechanical, it seems to be signal / computer related
Thanks
I think it is mechanically good, it should run, but it does not
I think it is a computer issue or a wiring issue. It has a “known good” ECM.
Does anybody know 1) Is there a new partial wiring harness that could be purchased and swapped in? 2) Is there a way to check a computer (ECM)? I feel there must be a shop with some sort of electronic tester or monitoring device.
Any other ideas? It does not seem mechanical, it seems to be signal / computer related
Thanks
Melting Slicks
Joined: Sep 2010
Posts: 2,422
Likes: 202
I have a 1987 Corvette. It will start, but it is a very hard start and runs rough. I took it to a shop. They replaced the intake (alternator, battery, “fixed” and exhaust leak). It does not have any smog stuff on it. It has the same issues it had before I took it to them.
I think it is mechanically good, it should run, but it does not
I think it is a computer issue or a wiring issue. It has a “known good” ECM.
Does anybody know 1) Is there a new partial wiring harness that could be purchased and swapped in? 2) Is there a way to check a computer (ECM)? I feel there must be a shop with some sort of electronic tester or monitoring device.
Any other ideas? It does not seem mechanical, it seems to be signal / computer related
Thanks
I think it is mechanically good, it should run, but it does not
I think it is a computer issue or a wiring issue. It has a “known good” ECM.
Does anybody know 1) Is there a new partial wiring harness that could be purchased and swapped in? 2) Is there a way to check a computer (ECM)? I feel there must be a shop with some sort of electronic tester or monitoring device.
Any other ideas? It does not seem mechanical, it seems to be signal / computer related
Thanks
(1) Replaced the intake gaskets. Or the actual intake.?
(2) Next you mentioned you have a known good ECU/ prom combo.
(3) asking about the wiring / possibilities.
Removing some SMOG components can create problems. Especially if the function is not turned off on the PROM / chip.. constant tables.
The garage should have validated the ECU system status. With a fairly easy ECU self diag test.
Is this a recent shop visit. You paid for a service.
The shop should have tested the ECU system as a starting point.
Unfortunately. A voluminous array of sensors, fuel pressure and related could easily create a hard start / rough running engine.
I just found this. By far the most thorough coverage ECU/ PCM diag coverage.
https://www.corvettebuyers.com/c4vettes/ecm.htm
over whelming. Can be confusing.
Your focus is the 87 ECU diag test code 1,2 CEL dash light. flash combo. Equals code 12 system ok.
Starting at the picture of a diagnostic port.
And this testing point.***Here is a typical 12 pin connector. (Some have more pins on the lower row than this example).
The pins you are interested in are those two pins in the top, right hand portion of the connector, designated pins "A" and "B".
Pin "B" is the diagnostic enable pin and pin "A" is ground. By grounding pin "B", you enable the diagnostic readout functionality of the ECM***
Thread Starter
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-(1) Replaced the intake gaskets. Or the actual intake.? It has all new gaskets and a stock intake. It had an edelbrock intake, but that deleted the 9th (cold start) injector – it is dead stock now.
(2) Next you mentioned you have a known good ECU/ prom combo. I have 4 or 5 ECM, but the one that is installed came out of a running 87 with a 4+3. I think it is good, but do not know how to check it
(3) asking about the wiring / possibilities. Could be the problem – all the smog stuff is here – in a box
(3) The garage should have validated the ECU system status. With a fairly easy ECU self diag test.
Is this a recent shop visit. You paid for a service. Yes – it is 3 weeks ago. I do not trust this shop because they seem to 1) FAILED TO GET IT RUNNING WELL 2) seems intent on selling me things I do not think I need (battery, altenator, etc)
(4) Unfortunately. A voluminous array of sensors, fuel pressure and related could easily create a hard start / rough running engine. Yes – this is the reason I am looking for an expert
(2) Next you mentioned you have a known good ECU/ prom combo. I have 4 or 5 ECM, but the one that is installed came out of a running 87 with a 4+3. I think it is good, but do not know how to check it
(3) asking about the wiring / possibilities. Could be the problem – all the smog stuff is here – in a box
(3) The garage should have validated the ECU system status. With a fairly easy ECU self diag test.
Is this a recent shop visit. You paid for a service. Yes – it is 3 weeks ago. I do not trust this shop because they seem to 1) FAILED TO GET IT RUNNING WELL 2) seems intent on selling me things I do not think I need (battery, altenator, etc)
(4) Unfortunately. A voluminous array of sensors, fuel pressure and related could easily create a hard start / rough running engine. Yes – this is the reason I am looking for an expert
Melting Slicks
Joined: Aug 2003
Posts: 3,395
Likes: 393
From: Lakeville MI
The ecm has very little to do with starting, other than providing the fuel pump signal and the injector pw for cranking fuel (which will vary as a function of coolant temperature). Once the engine is running, it's a different story.
Hard starting is all fuel related. Its likely a fuel/fuel pressure and/or injector issue assuming sufficient cranking rpm (battery/starter), airflow(throttle), a functioning ignition system with reasonable mechanical timing and a working coolant temperature sensor.
Here's an example log to demonstrate 1987-88 165 ecm signal behavior during cranking and after start-up and running: https://datazap.me/u/tequilaboy/cold...rk=110-119-143
Note: Don't be concerned with the values of any of the signals shown, since this is a custom tune with non-standard parameter tuning. YMMV.
Check the fuel pump relay and fuel pressure as a first step. Would also be a good idea to verify timing as well. A leaking fuel pressure regulator could also be the culprit. Don't be so quick to blame the ecm.
Hard starting is all fuel related. Its likely a fuel/fuel pressure and/or injector issue assuming sufficient cranking rpm (battery/starter), airflow(throttle), a functioning ignition system with reasonable mechanical timing and a working coolant temperature sensor.
Here's an example log to demonstrate 1987-88 165 ecm signal behavior during cranking and after start-up and running: https://datazap.me/u/tequilaboy/cold...rk=110-119-143
Note: Don't be concerned with the values of any of the signals shown, since this is a custom tune with non-standard parameter tuning. YMMV.
Check the fuel pump relay and fuel pressure as a first step. Would also be a good idea to verify timing as well. A leaking fuel pressure regulator could also be the culprit. Don't be so quick to blame the ecm.
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Joined: Oct 2007
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On fuel, I see the regulator under the right side of the Pentium. I understand what you are saying. Is there a way to measure the pressure (Like a gauge in the fuel rail I can screw into a shredder valve?) any recommendations on the valve type and a good value much appreciated!! I know it is getting fuel because I smell it (Maybe flooding)
Injectors were replaced a few miles back, but this was 5-7 years back. Is there a way to clean and/or test these?
Cranking (rpm) is very strong, it is better than most, so I do not think it is this issue.
mechanical timing – reset to TDC, without a running machine there is not much in the fine turning department I can do
example log to ecm signal – Very interesting. I do not think I understand enough about the signals to fully appreciate this, however two things 1) what equipment measures this and can I get one? Plug into the car and a Notebook? 2) I assume that if I have a value – it is good enough to start the process?
Leaking fuel pressure regulator – no external leakage (not sure about internal)
I did start putting the smog stuff back - missing 2 bolts (need to purchase) but it looks like a 90 minute job including dropping wrenches and figuring it out - mostly complete now and will be fully complete after I get a couple of bolts
Thank you!!! It helps to have somebody that thinks differently, particularly when my thoughts are not producing a solution
Injectors were replaced a few miles back, but this was 5-7 years back. Is there a way to clean and/or test these?
Cranking (rpm) is very strong, it is better than most, so I do not think it is this issue.
mechanical timing – reset to TDC, without a running machine there is not much in the fine turning department I can do
example log to ecm signal – Very interesting. I do not think I understand enough about the signals to fully appreciate this, however two things 1) what equipment measures this and can I get one? Plug into the car and a Notebook? 2) I assume that if I have a value – it is good enough to start the process?
Leaking fuel pressure regulator – no external leakage (not sure about internal)
I did start putting the smog stuff back - missing 2 bolts (need to purchase) but it looks like a 90 minute job including dropping wrenches and figuring it out - mostly complete now and will be fully complete after I get a couple of bolts
Thank you!!! It helps to have somebody that thinks differently, particularly when my thoughts are not producing a solution
Melting Slicks
Joined: Aug 2003
Posts: 3,395
Likes: 393
From: Lakeville MI
The data presented was obtained by scanning at 8192 baud with a Moates ALDL cable and ALDLdroid running on an old Samsung Galaxy tablet (wired connection with OTG adapter). I prefer to use a laptop pc with TunerPro since its a bit faster and more stable but the datastream is the same. The actual sample rate that you get when logging will vary a bit based upon the hardware with best case being around 10 samples per second.
The only point of the data in this example was to demonstrate how little the ecm has to do with startup. Most ecm related issues will result in a trouble code. If no codes are present, the ecm and its sensors are likely working properly, but its always useful to see the actual data for verification.
The only point of the data in this example was to demonstrate how little the ecm has to do with startup. Most ecm related issues will result in a trouble code. If no codes are present, the ecm and its sensors are likely working properly, but its always useful to see the actual data for verification.
