Service engine soon, but no codes?
Thread Starter
Instructor
Joined: Jul 2005
Posts: 140
Likes: 2
From: RI
Service engine soon, but no codes?
Hello, I finally have my 1988 back on the road after some time without it. I replaced the ecm, injectors, regulator, pump, egr valve, mat sensor, coolant sensor, egr sensor, cold start sensor, oxygen sensor, throttle position sensor, iac sensor, and plugs.
My car starts right up no problems and idles about 1,200 rpm until it warms up and then it settles down to the usual 700, but it is taking longer to do this.
Upon starting the service engine soon light comes on. I proceed to drive on the highway and get it up to 2000 rpm, and it goes off. If I decrease speed it will come back on. At idle it is on. If I brake suddenly it comes back on.
Car drives fine for the most part. The idle is steady. There are no codes.
Does anyone have any idea what the problem is, and why the light only shuts off at highway speeds with rpms over 1900?
Thank you,
Chris
My car starts right up no problems and idles about 1,200 rpm until it warms up and then it settles down to the usual 700, but it is taking longer to do this.
Upon starting the service engine soon light comes on. I proceed to drive on the highway and get it up to 2000 rpm, and it goes off. If I decrease speed it will come back on. At idle it is on. If I brake suddenly it comes back on.
Car drives fine for the most part. The idle is steady. There are no codes.
Does anyone have any idea what the problem is, and why the light only shuts off at highway speeds with rpms over 1900?
Thank you,
Chris
Team Owner
Joined: Sep 2001
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From: Athens AL
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If the SES light is on, then there is a code set.
Pull the car over and pull the code the next time it comes on.
Pull the car over and pull the code the next time it comes on.
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
Jumper ALDL pins A&B (upper right), start the engine and watch the SES flashes.
They should start out at 2Hz and, in a few minutes, drop to 1Hz and the On & Off cycles should be of equal duration. If these happen then the ECM is going closed loop and both the coolant temp sensor and the O2 sensor are ok; if not there could be a problem with one or both.
They should start out at 2Hz and, in a few minutes, drop to 1Hz and the On & Off cycles should be of equal duration. If these happen then the ECM is going closed loop and both the coolant temp sensor and the O2 sensor are ok; if not there could be a problem with one or both.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 1988redvette
I replaced the ecm, injectors, regulator, pump, egr valve, mat sensor, coolant sensor, egr sensor,
RACE ON!!!
Thread Starter
Instructor
Joined: Jul 2005
Posts: 140
Likes: 2
From: RI
Thanks for the positive comments. I did ground terminals a and b and only received code 12 continuously. The ecm is new and was definately a needed repair since the injectors were not receiving pulses. I replaced the injectors because I was running rich and the plugs were fouling.
I did take disconnect the battery, but the light came back on.
To the previous poster. I just received my FSM this past week. I appreciate the advice for I have only owned this car for two or three years and probably replaced a few unnecessary things, but I only see it from a positive standpoint. I enjoyed working on my car and the learning process. I am not complaining about spending the money for new parts. I rather spend a few dollars in parts and replace a few unnecessary things along the way than get raped by the dealership like I previously did.
I will figure it out. Thanks.
I did take disconnect the battery, but the light came back on.
To the previous poster. I just received my FSM this past week. I appreciate the advice for I have only owned this car for two or three years and probably replaced a few unnecessary things, but I only see it from a positive standpoint. I enjoyed working on my car and the learning process. I am not complaining about spending the money for new parts. I rather spend a few dollars in parts and replace a few unnecessary things along the way than get raped by the dealership like I previously did.
I will figure it out. Thanks.
Le Mans Master
Joined: Mar 2001
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From: Life is just one big track event. Everything before and after is prep and warm-up and cool-down laps
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St. Jude Donor '12
You probably have a historic code. They usually don't show up just jumpering the A&B pins. Find a friendly mechanic with a Tech1 and have them pull the historic codes.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 1988redvette
I enjoyed working on my car and the learning process. I am not complaining about spending the money for new parts. I rather spend a few dollars in parts and replace a few unnecessary things along the way than get raped by the dealership like I previously did.
I will figure it out. Thanks.
I will figure it out. Thanks.
RACE ON!!!
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Advanced
Joined: Dec 2006
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From: Jacksonville NC
What good is this?
Talk about throwing money at a problem... When the light comes on, a code is set. Have you studied your FSM?
RACE ON!!!
RACE ON!!!
Best to say nothing if this is best advise you can give to him and others who might have the same problem.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Generally, if it won't store a trouble code, the ECM is bad (and I know you replaced it). To be sure it's working, disconnect the TPS with it running. That should set a 22 and turn on the main fan (assuming coolant temp is below 226 degrees and the a/c is off) - ground the diagnostic or hook up a scanner and see what you get. Clear any codes that you do get. If no codes, try removing and reinserting the Prom.
The other possibility is an intermittent short to ground in the check engine light circuit. That's all the ECM provides - a ground for that light. Should come on (and go off shortly thereafter) everytime you start it. Disconnect the ECM - the light should go out. If not, there's a short to ground from the light to PIN A5 at the ECM.
Poke around - look around the diagnostic link - for frayed wiring or the attachment bolt poking through the harness. Wriggle the wires and see what the light does.
Slim possibility of something being wrong with the serial data circuit. You might suspect that if a scanner won't display data. Does the MPG and Range Function work?
The other possibility is an intermittent short to ground in the check engine light circuit. That's all the ECM provides - a ground for that light. Should come on (and go off shortly thereafter) everytime you start it. Disconnect the ECM - the light should go out. If not, there's a short to ground from the light to PIN A5 at the ECM.
Poke around - look around the diagnostic link - for frayed wiring or the attachment bolt poking through the harness. Wriggle the wires and see what the light does.
Slim possibility of something being wrong with the serial data circuit. You might suspect that if a scanner won't display data. Does the MPG and Range Function work?
Thread Starter
Instructor
Joined: Jul 2005
Posts: 140
Likes: 2
From: RI
Thread Starter
Instructor
Joined: Jul 2005
Posts: 140
Likes: 2
From: RI
Generally, if it won't store a trouble code, the ECM is bad (and I know you replaced it). To be sure it's working, disconnect the TPS with it running. That should set a 22 and turn on the main fan (assuming coolant temp is below 226 degrees and the a/c is off) - ground the diagnostic or hook up a scanner and see what you get. Clear any codes that you do get. If no codes, try removing and reinserting the Prom.
The other possibility is an intermittent short to ground in the check engine light circuit. That's all the ECM provides - a ground for that light. Should come on (and go off shortly thereafter) everytime you start it. Disconnect the ECM - the light should go out. If not, there's a short to ground from the light to PIN A5 at the ECM.
Poke around - look around the diagnostic link - for frayed wiring or the attachment bolt poking through the harness. Wriggle the wires and see what the light does.
Slim possibility of something being wrong with the serial data circuit. You might suspect that if a scanner won't display data. Does the MPG and Range Function work?
The other possibility is an intermittent short to ground in the check engine light circuit. That's all the ECM provides - a ground for that light. Should come on (and go off shortly thereafter) everytime you start it. Disconnect the ECM - the light should go out. If not, there's a short to ground from the light to PIN A5 at the ECM.
Poke around - look around the diagnostic link - for frayed wiring or the attachment bolt poking through the harness. Wriggle the wires and see what the light does.
Slim possibility of something being wrong with the serial data circuit. You might suspect that if a scanner won't display data. Does the MPG and Range Function work?
Also, I will make a few calls to see about getting it scanned. I might even buy the cable and use my laptop to see what is going on.
Thank you again for the knowledge. Only within the last couple years, since I bought this vette, is when I took an interest in mechanics. I am learning.
Chris
Melting Slicks
Joined: Feb 2004
Posts: 3,307
Likes: 111
From: Tucson AZ
Similar Problem
Chris,
I have a very similar problem with my 85. I have owned the car since new, but it has been stored for almost 15 years. I now have it running well except, it lights the "Service Engine Soon" light when ever I come to a stop. Mine sets a code 23, Manifold Air Temp (MAT) sensor voltage high. I changed the MAT sensor, but it still does it. I even sent the dash in to have it rebuilt thinking maybe the vehicle speed sensor input was not going through the dash properly. I have been checking wire bundles and even changed out the MAF sensor, but it still does it. Seems to run well, just keeps lighting the light when ever I come to a stop (engine idling). A few seconds after I drive away from the stop, the light goes out.
Please let me know what you find?
Vito
I have a very similar problem with my 85. I have owned the car since new, but it has been stored for almost 15 years. I now have it running well except, it lights the "Service Engine Soon" light when ever I come to a stop. Mine sets a code 23, Manifold Air Temp (MAT) sensor voltage high. I changed the MAT sensor, but it still does it. I even sent the dash in to have it rebuilt thinking maybe the vehicle speed sensor input was not going through the dash properly. I have been checking wire bundles and even changed out the MAF sensor, but it still does it. Seems to run well, just keeps lighting the light when ever I come to a stop (engine idling). A few seconds after I drive away from the stop, the light goes out.
Please let me know what you find?
Vito
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Highly probable you have a poor connection at the MAT. Essentially, the 23 means that the signal wire (Tan) has opened - though it could be on the ground side (Black). That ground is spliced into the TPS and CTS grounds, so usually if it's bad, you also get codes for these other Sensors. You don't have that. I'd try a new connector - sometimes replacement MAT's come with one. Just be sure to solder it in. Otherwise, take a good look at the one you got along with the wiring. I know it's difficult to get at since the Sensor is screwed into the bottom of the Plenum and that's sometimes the reason the connector gets messed up - you kinda fumble around with getting it disconnected and/or connected. Better (for me at least) to fork over the bucks for some new Runner gaskets and then R & R the Plenum.
Melting Slicks
Joined: Feb 2004
Posts: 3,307
Likes: 111
From: Tucson AZ
Code 23
You were right. The connector on the MAT was intermittent. Had to pull the plenum to get to it properly.
We took the connector apart and and re-pinned it. I put it all back together and used heat shrink to cover the connector and the lower section of the sensor and just installed the sensor with the extension wire connected.
Is there a source for new wire connectors?
Thanks
We took the connector apart and and re-pinned it. I put it all back together and used heat shrink to cover the connector and the lower section of the sensor and just installed the sensor with the extension wire connected.
Is there a source for new wire connectors?
Thanks
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
I will try disconnecting the TPS and see if it recognizes the code, in addition to poking around.
I am getting an intermittant with the SES light comming on with my '88 IROC-Z and it neither flashes any codes nor does it show any with an Auto-Xray scan.
Since your issue is consistant you can likely see the problem by scanning the ECM parameters during the occurance of the problem.
From your description it sounds like an issue between MAF & TPS readings (like the MAF is reading too high or too low for the throttle position that is seen) but the ECM should throw a code in such case.
This condition would show up under a scan.
You might probe pin A of the ALDL to see it +5Vdc is present with ign in Run and that gnd is present on pin B.
Le Mans Master
Joined: Aug 2003
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From: Magnolia Mississippi
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