Severe miss after driving for a short period of time

[TCHoward]

I recently purchased a 1990 ZR1. After driving the car for a short time there is a severe miss happening. The engine starts to shudder. When given gas the miss is still there. After a minute or so the miss goes away. Has anyone encountered this?

I have taken the car to Mr. Goodwrench and they said they found a vacuum leak which they fixed and the car still has the miss.

Miss may not be the correct word but that's what I'm using for lack of a better one.

[TCHoward]

Sorry folks for putting this thread in the wrong forum. Should have been in the technical forum I guess.

[tccrab]

Originally Posted by TCHoward

Sorry folks for putting this thread in the wrong forum. Should have been in the technical forum I guess.

Nah.. This is as good a place as any to get started.
First things first,
Welcome to the Registry!
You've come to the right place.


Intermittent problems are typically the hardest kind of problems to diagnose.
Is the SES light lit on the DIS?
Have you pulled the codes?
Is the car hard to start?
Do you get a lot of smoke on startup?
Are your secondaries functioning?
What work has been done to the car by the previous owners?
How many miles on the car?

Describe as accurately as you can what this miss/shudder sounds and feels like.
Does it miss at speed and at idle?
Does it go away when you open up the secondaries?

I'll cut some slack to your local Stealership, maybe the miss didn't occur when the car was in the shop and therefore they couldn't diagnose it.
But to be blunt, most Chevy shops have never seen a ZR1, much less have somebody that is trained and certified to work on the LT5.

More info will be needed before we can give a marginally accurate armchair diagnosis.

Where are you located?
There are half a dozen or so *really_really* good ZR1 tuners out there, maybe one is reasonably close to you.

[Kevin]

if you have a digtal cam that does video and a buddy get a vid of what the car is doing and upload and post here.

Some basic things to check:
how many miles are on the car
was it driven at all

[TCHoward]

Hey thanks for the welcome!

My ZR has 35K miles and it seems best I can tell from all of the records that I received with the car that most of the miles ere put on within the first couple of years. The last couple of years I would say that less than a 2000 miles have been put on it. I did see repairs for a couple of injectors.

I believe that the secondaries are opening. I can feel them kickin when getting on it. On another thread I did see that someone was experiencing the same symptoms and the response was that the injectors are probably shorting out when the motor gets to operating temperature. That seems to be the problem that I experience as it only happens when the motor is at operating temp. The strange thing is that it will go away in a couple of minutes after it starts. This morning I drove it to work and the temp outside was around 55 to 60 and sure enough after the engine got to temp it started missing. Couple of minutes later it stopped. Drove home this evening and it was around 85 and did not have a problem. Stumped.

By the way I live in Dallas TX. If anyone knows of a great tuner in this area please let me know.

I hate to go off half cocked and replace the injectors if they don't really need to be replaced. Secondly they are expensive!

Anyway I don't know if this shed any more light on the problem.

Thanks for all of your inputs!

[Kevin]

I know it's a bit of a haul but you may want to take a run over to cory hendersons shop in new braunfules http://www.hendersonperformance.com/index.html

[1990 quasar blue]

Originally Posted by TCHoward

By the way I live in Dallas TX. If anyone knows of a great tuner in this area please let me know.

http://www.hendersonperformance.com/

Originally Posted by TCHoward

I hate to go off half cocked and replace the injectors if they don't really need to be replaced. Secondly they are expensive!

You can test them with an Ohm meter. If they are bad Accell's can be had for 432.00 a set plus shipping.
http://www.autoshopracingengines.com/
Part # 150121 about 27.00 each and you'll need all 16

[TCHoward]

Thanks once again. I will give Henderson's a call. Just a couple of hours down the road from Dallas.

[tccrab]

Here's what you need to check the primary injectors without removing the plenum, courtesy of Marc Haibeck, "The ZR1 Specialist":
http://www.zr1specialist.com/HAT%20W...Resistance.htm
And since the write up is for a '91 and you have a '90 here's the correct page for a '90 from my Helms manual:


Good luck!

[A26B]

Originally Posted by TCHoward

.......it only happens when the motor is at operating temp. The strange thing is that it will go away in a couple of minutes after it starts. This morning I drove it to work and the temp outside was around 55 to 60 and sure enough after the engine got to temp it started missing. Couple of minutes later it stopped. Drove home this evening and it was around 85 and did not have a problem. Stumped.

Couple of additional possibilities to check out.

1. after the engine got to temp it started missing : ECM transitioning from open loop to closed loop. Couple of minutes later it stopped: Pull stored codes. This sounds like a possible O2 sensor/lead connection issue.

Start with simplest diagnostics first to eliminate common issues.

1. Pull stored codes & analyze as required. Start with lowest numerical code.
Then if problem not resolved, do the following.
2. Ohm out injectors (as previously recommended)
3. Ohm out paired cylinder wires/coils (as previously recommended)
4. Check fuel pressure, static, running cold & monitor as transitions from cold to oper temp.

There are numerous possibilities including sensors & ECM. Codes should be stored for most issues.

Good luck. Service manual are a real asset. You can find them on eBay for cheap used ones. A few inexpensive instruments are essential. Digital multi-meter, vacuum tester and fuel pressure gauge. Can save you a lot of guessing & dollars. Jump in, it's fun.