95 Corvette bad ECM?
Thread Starter
7th Gear
Joined: Aug 2016
Posts: 7
Likes: 0
95 Corvette bad ECM?
Hello all!
I have a 95 Corvette which has been giving me trouble. The car runs beautifully when I'm cruising but as soon as it's warmed up and I come to a stop light, it will start to have a bad miss. But this is very inconsistent. Sometimes at idle is will have a very small partial miss. Sometimes the car will idle fine with no issues. Another issue which has arisen recently is when I go to start the car, it will crank but not start. I will stop cranking and the second crank it starts up no problem. Again this is a more recent issue. Below is a list of stuff I have replaced on the car. I'm at a loss and I have been reading online that a bad ECM can cause inconsistent symptoms such as what I've been experiencing. But whoever had the car before me changed out the exhaust from the headers back (new CATS and mufflers) and have a small hunch the O2 sensors could cause trouble? But as inconsistent as it is, it has me stumped. What are your thoughts?
Things that I have replaced:
Optispark (Three times. All three had the same miss)
Spark plugs
Spark plug wires
Ignition Coil
Water Pump
Misc. Sensors
Fresh seals on the injectors (no vacuum leak)
I have a 95 Corvette which has been giving me trouble. The car runs beautifully when I'm cruising but as soon as it's warmed up and I come to a stop light, it will start to have a bad miss. But this is very inconsistent. Sometimes at idle is will have a very small partial miss. Sometimes the car will idle fine with no issues. Another issue which has arisen recently is when I go to start the car, it will crank but not start. I will stop cranking and the second crank it starts up no problem. Again this is a more recent issue. Below is a list of stuff I have replaced on the car. I'm at a loss and I have been reading online that a bad ECM can cause inconsistent symptoms such as what I've been experiencing. But whoever had the car before me changed out the exhaust from the headers back (new CATS and mufflers) and have a small hunch the O2 sensors could cause trouble? But as inconsistent as it is, it has me stumped. What are your thoughts?
Things that I have replaced:
Optispark (Three times. All three had the same miss)
Spark plugs
Spark plug wires
Ignition Coil
Water Pump
Misc. Sensors
Fresh seals on the injectors (no vacuum leak)
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
Scanner with snap shot functions or datalogging to catch what PCM is doing at time condition occurs. O scope opti reference and ICM EST circuits to besure square wave patterns.
Thread Starter
7th Gear
Joined: Aug 2016
Posts: 7
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Sorry forgot to mention codes. When I hook my scanner to it, no codes will be thrown. Which adds to the mystery.
Le Mans Master
Joined: Aug 2015
Posts: 6,119
Likes: 1,994
From: St. Charles MO
I think datalogging is your friend here. Since it only happens when it's warmed up, it sounds like it's happening after the PCM enters closed-loop mode. In that case, you're probably having a bad sensor somewhere that's tripping up the PCM - a sensor that is not read in closed-loop operation. These don't always throw codes, so datalogging can show you what sensor input is flatlined or going bat-**** crazy. The O2 sensors and the Coolant Temp Sensor (the one in the water pump, not the passenger-side cylinder head) are two likely culprits here.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
Was opti jumper / test harness replaced? Sometimes during the course continuous diagnosis the female terminals can spread causing intermittent or poor contact. Electrical supply or dealer could help with a terminals for testing. There are kits with test terminals and jumper wires. Somewhat expensive for DIYers.
Thread Starter
7th Gear
Joined: Aug 2016
Posts: 7
Likes: 0
Yes they were replaced and I confirmed that all contacts were good. Unless it's a loose connection after I plug them in?
Thread Starter
7th Gear
Joined: Aug 2016
Posts: 7
Likes: 0
I think datalogging is your friend here. Since it only happens when it's warmed up, it sounds like it's happening after the PCM enters closed-loop mode. In that case, you're probably having a bad sensor somewhere that's tripping up the PCM - a sensor that is not read in closed-loop operation. These don't always throw codes, so datalogging can show you what sensor input is flatlined or going bat-**** crazy. The O2 sensors and the Coolant Temp Sensor (the one in the water pump, not the passenger-side cylinder head) are two likely culprits here.
Le Mans Master
Joined: Aug 2015
Posts: 6,119
Likes: 1,994
From: St. Charles MO
I haven't specifically heard of that, but it certainly wouldn't surprise me in this day and age. I've certainly seen plenty of other replacement parts turn out to be bad, including a brand new ignition control module on my own car and two Autozone optis on a friend's 95.
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Verdad Gallardo
Team Owner
Joined: Jul 2004
Posts: 28,475
Likes: 3,291
From: Hartford WI
It's a mystery only because you assume that a code MUST be thrown if there is a problem. In reality, the code is thrown only when the sensor reports it is out of the "window". So as long as it is in the "window" or the values are plausible, life is good to the ECM. Think of it this way. If your bank statement says you are positive when in reality, you are not, you assume it is ok when you have NSF checks on the way.
Safety Car
Joined: Jan 2004
Posts: 4,549
Likes: 343
From: Glastonbury CT
When it gets warm closed loop just write down what the computer is actually supervising . Then take each item like coolant sensor and start checking with a meter.This can be done with the 1995 Helms manual. Also a visual check with your eyes is there oil running in back of the China wall or in the front? Check each spark plug wire for proper attachment to plug. (firmly pull on it) Was the coil replaced? Is there loads of oil near it for if when installed it must sit on a firm base of electrical gel for continuity.What about near the oxygen sensors is the wiring firmly making contact and what if any wires touching each other? What are you replacing your parts with , and are you testing prior to install? PM me if you have any more questions.