Inconsistent surging, missfires
Thread Starter
5th Gear
Joined: Oct 2012
Posts: 5
Likes: 0
Inconsistent surging, missfires
We've been working on our '90 Corvette for weeks now trying to get to the bottom of this.
When we got this thing it was missfiring like crazy and wouldn't hit overdrive if you hit the gas, had terrible power.
We replaced all the spark plugs and wires and it ran great... for a day.
Now it's back to misfiring and surging at low RPMS.
We think MAYBE it has to do with the performance distributor the previous owner put in it (looks like one of these http://www.autopartswarehouse.com/sk...%26+Drivetrain)
The Tach doesn't work when plugged in, it just maxes out.
Does anyone have any ideas or heard of this before?
When we got this thing it was missfiring like crazy and wouldn't hit overdrive if you hit the gas, had terrible power.
We replaced all the spark plugs and wires and it ran great... for a day.
Now it's back to misfiring and surging at low RPMS.
We think MAYBE it has to do with the performance distributor the previous owner put in it (looks like one of these http://www.autopartswarehouse.com/sk...%26+Drivetrain)
The Tach doesn't work when plugged in, it just maxes out.
Does anyone have any ideas or heard of this before?
Burning Brakes
Joined: May 2011
Posts: 838
Likes: 2
From: Louisville Ky
I didn't see any dostributor on that link.
you checked for stored codes? changed fuel filter? set timing? the factory manual has several pages for these symptoms.
the curious thing it is ran "great."
learn how to check the TPS -although yours is not adjustable. it can still be defective, not sure if intermittent.
unplug the MAP sensor, try it, see if you get a reaction. also can clean the 02 sensor by holding throttle at about 3000 for several minutes. I would ck a few plugs to see how they are burning. can be lean or rich A/F ratio.
also, injectors are always suspect unless changed recently. it ran ok with plug changed, so I suspect fuel, either rich or lean.
can remove vac line from FPR to see if fuel is leaking into plenum. not as likley, but only takes a minute to know.
you checked for stored codes? changed fuel filter? set timing? the factory manual has several pages for these symptoms.
the curious thing it is ran "great."
learn how to check the TPS -although yours is not adjustable. it can still be defective, not sure if intermittent.
unplug the MAP sensor, try it, see if you get a reaction. also can clean the 02 sensor by holding throttle at about 3000 for several minutes. I would ck a few plugs to see how they are burning. can be lean or rich A/F ratio.
also, injectors are always suspect unless changed recently. it ran ok with plug changed, so I suspect fuel, either rich or lean.
can remove vac line from FPR to see if fuel is leaking into plenum. not as likley, but only takes a minute to know.
Thread Starter
5th Gear
Joined: Oct 2012
Posts: 5
Likes: 0
I didn't see any dostributor on that link.
you checked for stored codes? changed fuel filter? set timing? the factory manual has several pages for these symptoms.
the curious thing it is ran "great."
learn how to check the TPS -although yours is not adjustable. it can still be defective, not sure if intermittent.
unplug the MAP sensor, try it, see if you get a reaction. also can clean the 02 sensor by holding throttle at about 3000 for several minutes. I would ck a few plugs to see how they are burning. can be lean or rich A/F ratio.
also, injectors are always suspect unless changed recently. it ran ok with plug changed, so I suspect fuel, either rich or lean.
can remove vac line from FPR to see if fuel is leaking into plenum. not as likley, but only takes a minute to know.
you checked for stored codes? changed fuel filter? set timing? the factory manual has several pages for these symptoms.
the curious thing it is ran "great."
learn how to check the TPS -although yours is not adjustable. it can still be defective, not sure if intermittent.
unplug the MAP sensor, try it, see if you get a reaction. also can clean the 02 sensor by holding throttle at about 3000 for several minutes. I would ck a few plugs to see how they are burning. can be lean or rich A/F ratio.
also, injectors are always suspect unless changed recently. it ran ok with plug changed, so I suspect fuel, either rich or lean.
can remove vac line from FPR to see if fuel is leaking into plenum. not as likley, but only takes a minute to know.
We did remove and clean the injectors. I'm not sure where to test the resistence though, I think its supposed to be 16ohm but i'm not sure where to stick the tester needles.
It's definitely running rich, it backfires if held at medium RPMS for a couple minutes (tach is broken but it seems about 3000).
the number 8 and 1 spark plugs had some unburned fuel on them when we changed them.
Where is the timing mark?
For the life of me we couldn't find it
EDIT: also no codes. Before changing the plugs we got a code saying 1,3,8 misfire, but gone now
Burning Brakes
Joined: May 2011
Posts: 838
Likes: 2
From: Louisville Ky
sORRY i MEANt ignition coil. The distributor underneath is the same brand that pairs with it.
We did remove and clean the injectors. I'm not sure where to test the resistence though, I think its supposed to be 16ohm but i'm not sure where to stick the tester needles
the number 8 and 1 spark plugs had some unburned fuel on them when we changed them.
[
EDIT: also no codes. Before changing the plugs we got a code saying
, but gone now
We did remove and clean the injectors. I'm not sure where to test the resistence though, I think its supposed to be 16ohm but i'm not sure where to stick the tester needles
.
It's definitely running rich, it backfires if held at medium RPMS for a couple minutes (tach is broken but it seems about 3000).
[
B]Where is the timing mark?[/B]For the life of me we couldn't find it
1,3,8 misfire
the injectors have a coil inside, so where do you think you would place the meter?
on the two electrical connectors where the harness goes? 
timing mark on balancer has to be marked with some color, white or red paint, whatever. also mark the 6* on the tab.
there is no 138 code.
what did you find when you checked the FPR vac line?
rich can be fpr leaking, coolant temp sensor defective, 02 sensor def. check at least one plug.
Thread Starter
5th Gear
Joined: Oct 2012
Posts: 5
Likes: 0
backfire is probably lean, or timing retarded. can be plug wires crossfiring.
the injectors have a coil inside, so where do you think you would place the meter? on the two electrical connectors where the harness goes?
timing mark on balancer has to be marked with some color, white or red paint, whatever. also mark the 6* on the tab.
there is no 138 code.
what did you find when you checked the FPR vac line?
rich can be fpr leaking, coolant temp sensor defective, 02 sensor def. check at least one plug.
the injectors have a coil inside, so where do you think you would place the meter?
on the two electrical connectors where the harness goes? timing mark on balancer has to be marked with some color, white or red paint, whatever. also mark the 6* on the tab.
there is no 138 code.
what did you find when you checked the FPR vac line?
rich can be fpr leaking, coolant temp sensor defective, 02 sensor def. check at least one plug.
Also not Code 138, i dont remember the code but when we had it checked they said it was cylinders 1, 3 , and 8 that were misfiring
Race Director
Joined: Jun 1999
Posts: 14,924
Likes: 9
From: Merritt Ils Fl
Josh I think something is not exactly correct in what you are being told.
Follow these instructions and pull codes, report them back please.
http://www.extreme-check-engine-ligh...%20Decoder.htm
As an afterthought I suggest you get a proper ID on the destributor as I think that will be the next question.
Follow these instructions and pull codes, report them back please.
http://www.extreme-check-engine-ligh...%20Decoder.htm
As an afterthought I suggest you get a proper ID on the destributor as I think that will be the next question.
Thread Starter
5th Gear
Joined: Oct 2012
Posts: 5
Likes: 0
i pulled the OBD1 for codes, Nothing at all. Just got code 12 (engine normal) constantly. I even jumped A and G to get codes on the speedometer but nothing there either.
haven't yet managed to get the FPR vac tube off i'm doing that right now (had to stop for ihop before we got mad)
haven't yet managed to get the FPR vac tube off i'm doing that right now (had to stop for ihop before we got mad)
Le Mans Master
Joined: Jan 2006
Posts: 5,620
Likes: 206
From: Orlando FL
On that year car I would either get the fuel injectors checked or install new set. They can cause all kind of problems and a code will never show up. You can ohm the coil of the injectors all day long but it will not relate to valve operation and proper a spray. At least then after that is resolved you will know the fuel delivery is not an injector problem.
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Thread Starter
5th Gear
Joined: Oct 2012
Posts: 5
Likes: 0
Yeah we're going to do the injectors soon.
As an update we noticed that when we unplugged the PCV the engine ran less rough.
We replaced it and the hose that connects it to the plenum since it was crumbling. The engine runs less rough but when we drove it and accelerated hard, it would lag at right about 30mph and then slowly climb up to 60.
The FPR vac looked clean and clear, no fuel in there or anything
As an update we noticed that when we unplugged the PCV the engine ran less rough.
We replaced it and the hose that connects it to the plenum since it was crumbling. The engine runs less rough but when we drove it and accelerated hard, it would lag at right about 30mph and then slowly climb up to 60.
The FPR vac looked clean and clear, no fuel in there or anything