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Misfire at 1500 rpm, 70% throttle

A

Adam...

Guest
I have a strange misfire on my new purchase (92 LT1). It will give me a rather hard misfire at about a 70% throttle at 1500 rpm. Below that throttle position it gets better and even goes away at very light throttle. Above that throttle position it even seems to be less severe. Also, there does seem to be a little lumpyness at idle which would indicate a random misfire. However, everywhere else in the rpm range it is smooth. I am going to throw new plugs and plug wires at it to see what happens but I am curious if this problem rings a bell to any other members and what you did to eliminate it?

Thanks,
Adam
 

69MyWay

Well-known member
Joined
Jul 3, 2001
Messages
4,364
Location
Auburndale, Florida
Corvette
1969 Killer Shark
Adam,

Welcome to CAC! You could be seeing early signs of a bad opti-spark.

C4C5 specialist is are local expert. He might be able to better tell you have to diagnose it.

However, a bad coil wire will do the same, so you are on the right track by doing a regular tune up.

Let us know how it runs after that.

You might also want to test your WOT fuel pressure. You could have a bad regulator or bad fuel pump.
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Welcome

Pop the hood, and roll up yer sleeves,ITS WRENCHIN TIME.

How many miles on the car?

Take a look behind the water pump, that THING where all the plug wires go is the OPTI-CRAP ( a high probablity failure part)

Its the sensor for the spark timing & the distributer all mushed into 1.

Do you have the orig water pump in there?
Is there any seepage of water behind or around the pump?

I agree with Chris.. do a tune up. replace wires ( if in budget ) re test. Make sure the wires all tight and seated home & re test.

Mike
 
A

Adam...

Guest
The car has about 81000 miles on it and it looks like the original water pump. I would not be surprised if it has the original plugs in it. I cannot find any leaks around the pump and I don't think this car has seen a lot of rain (from what the previous owner states). This is a Florida car and seems to have been pretty pampered. (I know it is a Florida car as I just drove it from there all the way to Michigan.)
 
V

Velo

Guest
Misfire.

I agree with all the suggestions about the Optispark etc.
I do have one question the picture about 1500RPM at 70% throttle baffle me.
Is this from an idle to go directly to 70% throttle and then there is a miss at 1500RPM.
Again I agree go the cheaper route first because the waterpump and Optispark will set you back a tidy sum. Good Luck.
 
A

Adam...

Guest
I will happen whenever you get to that point; From a standstill or from steady state cruising.
 
D

David-F

Guest
Are you sure it's a missfire? or is it just a vibration? Is your car an auto? Can you get it to happen while power braking? Seeing how you plan on changing the plugs and wires you migh as well go ahead and replace the water pump and opti-spark distributor. I know this sounds like just throwing money around by history shows that the distributor is good fo 60 to 80,000 miles and if you have the original plug wires which means they have the cylinder number on them then there is a good chance that the cap on the distributor has already been stresed and am sure there may have been some random miss fires in the cap already and it would be a real shame to go a head with the plugs and wires onlly to have to go back in and do the distributor, and it makes no sence to do the distributor and not replace the water pump, I bet it's on it's way out anyway. If you want you can get a spray battle and fill it with salt water and spray down the wires at night when it's warm and watch for a light show but it would make no sence if you know you have the original wires. Now it's possible you could have some other problem like bad EGR valve but to just rule it out just take and pull the vaccumn hose from the EGR valve and drive the car if it improves then there may be a problem with either the EGR solenoid or EGR valve. Also you do need to remove the EGR valve and make sure it is able to close all the way over time there can be carbon deposets that can keep it fron closing all the way, you may be able to clean it but it just might be batter to just replace it.

David Fulcher
 
A

Adam...

Guest
Well you are right about the EGR valve. I put a vacuum tester on it to pull a vacuum to see if it would open and it appears the diaphram leaks as I could not pull a vacuum (and hense open the valve). Is this really enough to cause such a bad misfire at that specific rpm/load location? I will replace it but I suspect there is something ignition related as well.
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
The application of vacuume

OPENS the valve. If your diaphram is blown
the EGR will remain shut, but it will cause a VAC leak.

Mike
 
D

Drewser

Guest
Rrrrrufff

LOVE the poochie, Mike:) It seems my little yappy b!tch has set a trend.
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
The CAC is going

To the DAWGS!

Mike
 
V

vettepilot

Guest
Adam,
I fully agree with the post stating that it's probably the opti-spark. Was the engine nice and clean when you purchased the car? The previous owner or dealer may have cleaned the engine bay with high pressure water, that spells death to the optispark distributor cap because it fills with water, even the vac line can't suck it all out. The miss that you describe really sounds like what I experienced in my 93 when the opti-spark got loaded with water after I boneheadedly cleaned my engine compartment with water. It would idle rough, miss under heavy load, but smooth out at the higher RPMs; it was still missing, you just can't feel it as easily. And it won't just go away, it eats the dist. cap and contacts due to the arcing between contacts.
At this point you could try several things:
1. Take to a tech, have him computer test the system, (about
$100.00) in my area, it should reveal the problem.

2. Trouble shoot the system yourself, this requires isolating the
problem, replacing plug wires, checking plugs etc. takes time
and money to do this.

3. Remove the opti-spark, remove the distributor cover, check
for carbon tracking, moisture tracking, corrosion etc. If any of
the before mentioned are found, you might as well replace the
unit now because you will have to in the future. Also will give
you the chance to really check the rear seal of the waterpump.
Replace the pump if it looks at all suspicious.
These engines because of the computers, and digital components usually run at 100% or not at all. Seems like the opti-spark is the one component that sometimes will work at less than 100%, but not for very long.
Good Luck!
Stan
 
D

David-F

Guest
Unfortunitly a computer test may not show much, if it was a 96 you could graph missfire. There may be some computer test equiptment out there that may show some things but even that could be missleading. First thing the EGR will not open with a vacumn pump if the engine is idling that is one of the quick checks now if it was tested with the engine not running then it is not working and would need to be replaced. But with this said you would have to remove the EGR valve to see if it is leaking past the pintel. An yes a EGR that comes on when it should not can cause a what feels like a slight loose of power under light throttle but over a certian amount of throttle it will not come on. Again you do need to take a look at your spark plug wires and if they have the cylinder numbers on the wires like 1 or 2 or 3.....then you do need to replace them and while you are at it the plugs and distributor and water pump unless you have evedence that it has been changed in the past few years. You have to start with the basic it's just that at the mileage that is on your car somethings need to be done anyway, if you want to wait untill after the plugs and wires have been changed before thinking about the opti-spark that's fine but if you have the original wires on the car they need to be changed even if it's not the problem at this time they soon will and a bad plug wire can take out a very expensive cat converter and or a distributor.

David Fulcher
 

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