94 surging/missing problem

[copper_94]

hello

i have a problem with my 94 that is puzzling me.

6speed
140k miles
no ses light
service ride control (z07 option) light is on, but that is a bad servo that
i know about
no other indicators on
new battery
alternator normal output
all other temp indicators in normal ranges

the symptoms started a few months ago. in top gear at high way speeds, it
will seem to miss when a demand is placed on the engine (going up a hill,
passing, etc). in the ensuing months, it seems to have gotten worse, in that
it now:

at idle (which used to be around 800rpm), it would start to hunt, going from
800-2000, with no throttle input. during around town driving, say in 4th at
45, the engine would speed up so the car would accelerate, with no throttle
changes being done by me. the pedal doesn't move under my foot, so it's not
a spring, i believe.

i replaced the iac valve and airfilter at this point. when the old one iac
was removed, it had a lot of crud on it. i cleaned out the opening to the
throttle body, but didn't take it off for a complete cleaning. i also
replaced the water temp sensor, as i noticed the harness connector was
cracked. actually, the plug had fallen out and the problem went from occasionally to having no power at all, stranding me. when i plugged it back in, the problem went back to the occasional problem again.

the problem has now changed, in that the idle has dropped to an abnormal low
of 400 and seems to almost stall. it still has the missing and bucking at
constant speeds, and also misses pulling away from a stop.

i took it to my local mechanic, whom i've had for about 20 years. he noticed
that every time it surges, the service ride control light changes intensity.
he seems to feel that the computer is bad and is affecting the timing
somehow, and said i should take it to the chevy dealer.

could this be a fuel delivery problem (fuel pump, filter, bad injectors)?

are there any other sensors that this could be, or does this sound familiar,
and if so, are there tests for them so i don't have to start throwing
expensive parts at the problem?

thanks,
charlie

 

[gmjunkie]

hello

i have a problem with my 94 that is puzzling me.

6speed
140k miles
no ses light
service ride control (z07 option) light is on, but that is a bad servo that
i know about
no other indicators on
new battery
alternator normal output
all other temp indicators in normal ranges

the symptoms started a few months ago. in top gear at high way speeds, it
will seem to miss when a demand is placed on the engine (going up a hill,
passing, etc). in the ensuing months, it seems to have gotten worse, in that
it now:

at idle (which used to be around 800rpm), it would start to hunt, going from
800-2000, with no throttle input. during around town driving, say in 4th at
45, the engine would speed up so the car would accelerate, with no throttle
changes being done by me. the pedal doesn't move under my foot, so it's not
a spring, i believe.

i replaced the iac valve and airfilter at this point. when the old one iac
was removed, it had a lot of crud on it. i cleaned out the opening to the
throttle body, but didn't take it off for a complete cleaning. i also
replaced the water temp sensor, as i noticed the harness connector was
cracked. actually, the plug had fallen out and the problem went from occasionally to having no power at all, stranding me. when i plugged it back in, the problem went back to the occasional problem again.

the problem has now changed, in that the idle has dropped to an abnormal low
of 400 and seems to almost stall. it still has the missing and bucking at
constant speeds, and also misses pulling away from a stop.

i took it to my local mechanic, whom i've had for about 20 years. he noticed
that every time it surges, the service ride control light changes intensity.
he seems to feel that the computer is bad and is affecting the timing
somehow, and said i should take it to the chevy dealer.

could this be a fuel delivery problem (fuel pump, filter, bad injectors)?

are there any other sensors that this could be, or does this sound familiar,
and if so, are there tests for them so i don't have to start throwing
expensive parts at the problem?

thanks,
charlie

I'd Start Right Here!!!!!!This is something that NoBody checks if the SES light is not on!!! Be my guess that the EGR has a chunk of Carbon!!!

http://www.corvetteactioncenter.com/forums/showthread.php?t=94799

 

[gmjunkie]

Heres a photo,the EGR is at the back of the intake !!I put a Green Dot on this 94 EGR so you could see it!!!!

 

[copper_94]

well, the egr was pretty carboned up, but i replaced it and still have exactly the same problem.

 

[John Robinson]

Just for fun you could check the TPS voltage. The TPS tells the ECM how much fuel to deliver to the cylinders.

Oh and you could also check the compression. After all the plugs are no problem to remove to put a compression tester in the plug hole

 

[omega1940]

Surging

I'm certainly no expert, but I have had similar problems with all 3 of the Corvettes I have owned, all with Mass Air Flow sensors. I would probably start by replacing the Throttle Position Sensor or TPS. You can do it yourself, but it is a bit difficult because you have to make sure the valve is in the same position as the old one or the idle will be way to fast when you first start it up. The O2 Sensor(s) could also be bad. It sounds like the ECM, electronic control module, is constantly searching for the right air/fuel mixture, and because one of the sensor or the IAC are bad it just keeps boosting the fuel/air to high then adjusting it down, etc.

Some other expert will probably give you a more specific answer, but the above are usually the culprit.

Best of luck.

Mike

 

[copper_94]

thanks. the tps may be bad, but i have no means to measure and don't even know where it is. i also have a 96 that i could take one off to try; is the tps the same part number for 94 and 96?

the o2 sensors could certainly be bad. they never been replaced as long as i've had the car (10+ years).

i replaced the iac first, since i had this exact problem on my older 89 and that fix solved it there.

 

[Toms007]

I am waiting on what the culprit is here. I don't have the problem, but someday I might and it would be nice to know what is causing it. Junkie's guess on the EGR was exactly what I thought of when I read your initial post. When it wasn't that, I was shocked. Keep us posted.

 

[KANE]

well, the egr was pretty carboned up, but i replaced it and still have exactly the same problem.

Opti-Spark strikes again...

Below are links to lots of threads that deal with Opti-Spark
http://www.corvetteactioncenter.com/forums/search.php?searchid=561046
http://www.corvetteactioncenter.com/forums/showthread.php?t=32486&highlight=optispark+miss

Vette did a great article on it a while back-
http://www.vetteweb.com/features/vemp_0611_corvette_lt1_msd_distributor/diagnosis_101.html

Diagnosis 101
Think your Opti is on its way out? Some common traits exhibited by a faulty OptiSpark include:

•Excessive cranking required to start
•Starts but dies immediately
•Rough idle with backfiring or sputtering
•Poor performance with car warmed up
•Poor high-rpm performance
•Poor gas mileage
•Black exhaust
•Weak plug-wire spark
•Car suddenly dies and won't restart

Essentially, there is moisture n the distributor and it throws off the distributor and it misses.

I'd replace it with a MSD unit. Much better quality than OEM.

 

[gmjunkie]

Opti-Spark strikes again...

Below are links to lots of threads that deal with Opti-Spark
http://www.corvetteactioncenter.com/forums/search.php?searchid=561046
http://www.corvetteactioncenter.com/forums/showthread.php?t=32486&highlight=optispark+miss

Vette did a great article on it a while back-
http://www.vetteweb.com/features/vemp_0611_corvette_lt1_msd_distributor/diagnosis_101.html



Essentially, there is moisture n the distributor and it throws off the distributor and it misses.

I'd replace it with a MSD unit. Much better quality than OEM.

It very well could be the Opti or even the coil braking down!!!Coil wire or Plug wires!!!! I would think for a Oxy sensor,TPS or Mass air it would most certainly throw some codes!!!!! PS Have you ever ran the battery down,and then just charged it up without disconnecting it!!!!Sometimes after charging the ECM will get all Confused and needs to be Rebooted!!! Try taking the neg cable off of the Battery for 5 min,and then hook it back up and see if that changes anything!!

 

[John Robinson]

TPS 101

Sorry about using TPS (Throttle Position Sensor) in my replay. I remember how confusing all lthe abreviations were to me when I first joined the CAC. To help you understand what the TPS is and how it works I have prepared the followin from the GM service manual. (Best money I have ever spent on the car) Good luck on finding your problem. Remember no question is stupid only the ones not asked are.

Throttle Position Sensor (TPS)

Copied from 1993 Service Manual
​

The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

 

[gmjunkie]

I'm not totally sold that the EGR is not the problem!!! It don't take a very big piece of carbon,Even on a NEW one!!!! There is still carbon in the Intake passages, 1 little piece will stick even a NEW EGR part way OPEN !!!!:eyerole You may not even be able to see it part way open with your eyes!! If the old one was Gooked up,Theres more where that came from,and it will stick again!!! New or Not!!!! hnoes

Unless you do this,to keep it broke up small enough to go through the EGR!!

http://www.corvetteactioncenter.com/forums/showpost.php?p=812161&postcount=4

 

[copper_94]

Sorry about using TPS (Throttle Position Sensor) in my replay. I remember how confusing all lthe abreviations were to me when I first joined the CAC. To help you understand what the TPS is and how it works I have prepared the followin from the GM service manual. (Best money I have ever spent on the car) Good luck on finding your problem. Remember no question is stupid only the ones not asked are.

Throttle Position Sensor (TPS)

Copied from 1993 Service Manual
​

The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

thanks. i did find a picture of where it was on the net, and put a dvom on it this morning. i don't have a test harness to see what voltage it is putting out at different throttle positions, but it does show a constant change in ohms through the throttle range.

here's a weirdness: i attempted to read the dtc codes but it didn't seem to go into the maint mode. i shorted pins 4&12, turned on the ignition, and the speedo read out 0. i didn't get any c12 like it was supposed to do. the only difference was the ses light stayed on until i cycled the key to off, pulled the paper clip, and turned the key on.

is there a mode that it has to be in before it will read out codes?

 

[copper_94]

I'm not totally sold that the EGR is not the problem!!! It don't take a very big piece of carbon,Even on a NEW one!!!! There is still carbon in the Intake passages, 1 little piece will stick even a NEW EGR part way OPEN !!!!:eyerole You may not even be able to see it part way open with your eyes!! If the old one was Gooked up,Theres more where that came from,and it will stick again!!! New or Not!!!! hnoes

Unless you do this,to keep it broke up small enough to go through the EGR!!

http://www.corvetteactioncenter.com/forums/showpost.php?p=812161&postcount=4

does the egr work by opening when the engine gets to temp? on the old one that is off, i cleaned it up pretty well with kerosine, and when dried and in the closed position, i can blow in the valve tube and it will leak a little bit.

the spring appears ok in the old one, as it moves ok when i press in on it, and the diaphram is whole because it blows out the vacuum tube.

what would happen if i drove the car with the vacuum disconnected to the egr?

 

[gmjunkie]

here's a weirdness: i attempted to read the dtc codes but it didn't seem to go into the maint mode. i shorted pins 4&12, turned on the ignition, and the speedo read out 0. i didn't get any c12 like it was supposed to do. the only difference was the ses light stayed on until i cycled the key to off, pulled the paper clip, and turned the key on.

is there a mode that it has to be in before it will read out codes?

Which dose your car have,OBD 1 or OBD II ???? Some 94 had OBD1 some had OBDII!!
Which dose it have??? A 12 pin ALCL connector or a 16 pin ALCL connector??

 

[copper_94]

mine has a 16 pin alcl connector, but not all the holes are populated with pins. i saw the 12 pin style connector in the manual and it didn't even describe the 16 pin connector.

 

[copper_94]

btw: i see at http://corvetteactioncenter.com/forums/showthread.php?t=14341&highlight=12+dtc that MY94's have this ability. mine was an early build as it was a press car, so probably doesn't. guess i'll have to find a shop with a scanner.

thanks,
charlie

 

[John Robinson]

btw: i see at http://corvetteactioncenter.com/forums/showthread.php?t=14341&highlight=12+dtc that MY94's have this ability. mine was an early build as it was a press car, so probably doesn't. guess i'll have to find a shop with a scanner.

thanks,
charlie

Advance Auto Parts will scan your car for free.

 

[copper_94]

Re: 94 surging/missing problem FOLLOWUP

Opti-Spark strikes again...

Below are links to lots of threads that deal with Opti-Spark
http://www.corvetteactioncenter.com/forums/search.php?searchid=561046
http://www.corvetteactioncenter.com/forums/showthread.php?t=32486&highlight=optispark+miss

Vette did a great article on it a while back-
http://www.vetteweb.com/features/vemp_0611_corvette_lt1_msd_distributor/diagnosis_101.html



Essentially, there is moisture n the distributor and it throws off the distributor and it misses.

I'd replace it with a MSD unit. Much better quality than OEM.

short story: bingo.

long story: i took it into the dealer after my mechanic didn't find the problem. he didn't want to start just throwing parts at it, and didn't have any onhand to do any testing with.

the dealer at first diagnosed the missing problem as a couple of bad spark plug wires, and possibly bad plugs. replaced all wires/plugs.

the surging problem he claimed was a MAS with baked on oil from the k&n air filter. solvent cleaned the MAS.

turned it back to me, and within 1/2 mile was doing the same thing. went back, took the service guy for a test drive and nearly put him through the windscreen it was missing so bad.

they kept it for a day and said they couldn't reproduce the problem. went back and took the manager for a test drive, and bucked him around some.

i don't know what they did to diagnose, but called and said the optispark was bad. took a couple of days to get one. took out the water pump and found that it was on it's way to selfdestruction, missing some bearings and a rusty optispark. replaced both. wonder where the bearings will turn up in the future?

runs like a champ now, as far as i can tell with 20 or so miles on it. lots lighter in the wallet, and i can't recommend midway chevy in phoenix for any service at all.

thanks for the help,
charlie

 

[copper_94]

Which dose your car have,OBD 1 or OBD II ???? Some 94 had OBD1 some had OBDII!!
Which dose it have??? A 12 pin ALCL connector or a 16 pin ALCL connector??

it has an obd2 connector, but doesn't respond to the obd2 reader i borrowed at checker auto. it says there's nothing there.