1987 Occasional Check Engine Light
Thread Starter
Heel & Toe
Joined: Apr 2020
Posts: 21
Likes: 6
From: Massachusetts
1987 Occasional Check Engine Light
Hello all,
I have a 1987, and lately the Check Engine Light will sometimes come on when I first start it up, doesn't sound or drive differently, and after I drove 20 minutes to a car show, let it sit there for a few hours, and when I drove it home it did not come on at all. Drove it a few times locally since then and it has been fine, until today it came on again when I started it. I have to take it to my mechanic soon to get inspected once the RMV sends me plates and registration, but in the interim thought I'd ask and see if anyone has encountered this issue before.
Thank you!
I have a 1987, and lately the Check Engine Light will sometimes come on when I first start it up, doesn't sound or drive differently, and after I drove 20 minutes to a car show, let it sit there for a few hours, and when I drove it home it did not come on at all. Drove it a few times locally since then and it has been fine, until today it came on again when I started it. I have to take it to my mechanic soon to get inspected once the RMV sends me plates and registration, but in the interim thought I'd ask and see if anyone has encountered this issue before.
Thank you!
Safety Car
Joined: May 2001
Posts: 4,754
Likes: 1,643
From: Fairfax Virginia
This may be a long shot.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
Jump the A to B port on the diagnostic connector and get the code that is the first step.
If I have to take a guess it would be the MAS power relay or the burn-off relay. Those will pop on at the first hint of a problem but if the condition is only intermittent the car will run fine if the condition fixed itself. My 1988 firebird with a MAS and TPI would turn on the check engine light as soon as it was started but run fine, it was the MAS burn off relay. I would immediately turn the car off and restart and no light, but a code was set so I replaced the relay and problem solved.
This is my guess but check the code.
Last edited by bjankuski; Sep 2, 2021 at 05:03 PM.
Safety Car
Joined: Aug 2002
Posts: 3,842
Likes: 230
From: SANTA CRUZ CA
Safety Car
Joined: Aug 2002
Posts: 3,842
Likes: 230
From: SANTA CRUZ CA
MORE CODE INFO (click upper left corner to see more pages):
https://www.corvettebuyers.com/c4vettes/codes.htm
https://www.corvettebuyers.com/c4vettes/codes.htm
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Instructor
Joined: Aug 2004
Posts: 203
Likes: 12
From: Caldwell Texas
My 88 does this. I checked the code and it says check the MAF. It will come on sometimes right after you start it, then the next start, it doesn't come on, and it might not come on for 15 or 20 starts.
Update:
Since yesterday I have been tinkering with my car, a 1988, auto. 63,000 miles. I bought it last Dec. been doing basic things to it. since then, mainly cleaning it up and uninstalling a car alarm. A couple months ago I changed the plugs, wires, cap, rotor and coil. Never got around to checking the timing. Ever since then, I would get the occasional SES light when starting it up, the same symtoms the OP, lloyd88 is describing. I took the time yesterday to check the timing. It was at 2*, so I set it to the 6* it needed to be. Took it out for a spin and then agin for some odds and ends. i have been reading post that say they run theirs at 8* or 10* with no problems , so today I reset it to 10*. In doing this, there is a lot of starting and never once did the SES light come on since setting the timing to the correct 6*. Is there somethhing to this??
lloyd88, you might check your timing to see where it is at. Not being advanced enough culd be causing the problem.
Update:
Since yesterday I have been tinkering with my car, a 1988, auto. 63,000 miles. I bought it last Dec. been doing basic things to it. since then, mainly cleaning it up and uninstalling a car alarm. A couple months ago I changed the plugs, wires, cap, rotor and coil. Never got around to checking the timing. Ever since then, I would get the occasional SES light when starting it up, the same symtoms the OP, lloyd88 is describing. I took the time yesterday to check the timing. It was at 2*, so I set it to the 6* it needed to be. Took it out for a spin and then agin for some odds and ends. i have been reading post that say they run theirs at 8* or 10* with no problems , so today I reset it to 10*. In doing this, there is a lot of starting and never once did the SES light come on since setting the timing to the correct 6*. Is there somethhing to this??
lloyd88, you might check your timing to see where it is at. Not being advanced enough culd be causing the problem.
Last edited by c.w.moss; Sep 6, 2021 at 03:01 PM.
Thread Starter
Heel & Toe
Joined: Apr 2020
Posts: 21
Likes: 6
From: Massachusetts
This may be a long shot.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.
Safety Car
Joined: May 2001
Posts: 4,754
Likes: 1,643
From: Fairfax Virginia
I would replace the BOTH MAF relays while you are at it. This will ensure the MAF has the best chance of working properly again. I never replace just one relay at a time, replace both any time you have trouble with your MAF or have replaced your MAF sensor.
By the way I am a big believer in replacing Oxygen sensors that are over two years old or have 50k miles on them. The O2 is one of the most important devices on your EFI system and since it is cheap I will suggest replacing them to better the economy and performance of the Corvette. If an O2 has been installed five years ago and the car was not driven at all it is still a five year old Oxygen sensor and needs to be replaced.
The suggestion by ihatebarkingdogs concerning the MAF not having performed a "Burn Off" on previous shut down makes sense. That is clearly a possibility. I have seen the occasional light myself on my 1988 C4 but not since I replaced the MAF sensor and both relays only a couple years ago.
By the way I am a big believer in replacing Oxygen sensors that are over two years old or have 50k miles on them. The O2 is one of the most important devices on your EFI system and since it is cheap I will suggest replacing them to better the economy and performance of the Corvette. If an O2 has been installed five years ago and the car was not driven at all it is still a five year old Oxygen sensor and needs to be replaced.
The suggestion by ihatebarkingdogs concerning the MAF not having performed a "Burn Off" on previous shut down makes sense. That is clearly a possibility. I have seen the occasional light myself on my 1988 C4 but not since I replaced the MAF sensor and both relays only a couple years ago.
Instructor
Joined: Aug 2004
Posts: 203
Likes: 12
From: Caldwell Texas
Because the CEL comes on right after a start, I'm going with Code 36. Code 36 will set if burn-off did not occur on the previous shutdown. The CEL will be illuminated on the next start.
bjanskuski suggested that it could be the MAF power relay or the MAF burnoff relay. If either of these relays become resistive or fail, Code 36 will set.
It's NOT the O2 sensor, and it is not timing-related. Neither of these will set a CEL on start-up. Actually, very few normal failure conditions will.
bjanskuski suggested that it could be the MAF power relay or the MAF burnoff relay. If either of these relays become resistive or fail, Code 36 will set.
It's NOT the O2 sensor, and it is not timing-related. Neither of these will set a CEL on start-up. Actually, very few normal failure conditions will.
Thread Starter
Heel & Toe
Joined: Apr 2020
Posts: 21
Likes: 6
From: Massachusetts
This may be a long shot.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.
There is a timer function in the ECM that monitors how long the Engine has been running in "Open Loop". If your Oxygen Sensor is over 24 months old (or 50K miles) it is time for a new one.
Oxygen sensors were supposed to warm up to ~600* (f) within a short time of idling. Once the O2 warms up it starts generating a signal. The signal oscillates between lean and rich very very quickly. Older O2 signals can start to slow down the signal and make it narrower than it needs to be for your engine to run properly. If the signal is not there when expected the ECM might trigger a SES light.
When you start the Corvette up and the SES light turns on, was the engine still in "Open Loop" or "Closed Loop"? This is important.
When a engine starts up in "Open Loop" and then warms up and then goes into "Closed Loop" it is watching three primary sensors, The Oxygen Sensor, The Coolant Temperature Sensor and the Mass Air Flow Sensor (or MAP) will be used by the ECM to calculate the quantity of fuel that will be injected and the timing.
If your engine is having a hard time going into "Closed Loop" my first guess would be the Oxygen sensor. If it is not the O2 then I would start cleaning connections both 12 V and ground under and near the dashboard. Having access to a Scanner would save you a lot of time troubleshooting. I am leaning that direction myself as the "Live" information is wonderful when looking for an errant sensor.