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.:confused:

[TCHoward]

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

[tccrab]

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.
:-D

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!:mrgreen:

[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]

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/

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%20Web/articles/FI%20Resistance.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:
http://i71.photobucket.com/albums/i159/tccrab/ZR1_Primary_Injectors.jpg

Good luck!

[carter200]

Contact EES at www.acsvette.com (http://www.acsvette.com) and Mike will fix it right:thumbsup: By the way, welcome:handshak:

[Kevin]

carter,I think that maybe a bit of a haul the this guy. Cory will do him right

[TCHoward]

Thanks Tom C. Never done diagnostics like this but hey there is a first time for everything!

[TCHoward]

I went out to ACS Vette but could not find where they are located. Anyone know?

[tccrab]

Thanks Tom C. Never done diagnostics like this but hey there is a first time for everything!
Tom:

It looks and sounds harder than it really is.
A fuel injector is a simple beast actually, not hard to understand at all.
It has a needle valve (also called a pintle) that is spring loaded into the injector body. There is a solenoid coil that acts as an electro-magnet which pulls up on the pintle when there's 12v's put through it. When the electromagnet pulls up on the pintle, the gas comes out and when the pintle goes down the gas stops.
See? Simple.
You're simply measuring Ohms of the solenoid coil in the fuel injector.
The typical failure is that the coil gets shorted because ethanol in the gasoline eats through an internal seal and starts chewing up the insulation on the solenoid coil. When that happens, the coil can no longer hold the pintle up long enough and the fuel injector does not squirt enough gas or no gas at all. As the conditions in the fuel injector changes, i.e., temperature, pressure, phase of the moon, rings around Uranus, the coil sometimes works, and sometimes not.
Viola!! Intermittent missfire.
The coils should measure about 12-15ohms depending on temperature and other things like different resistance in the circuit because of different wire length.
But that's really not a biggie, you're looking for a BIG difference.
If when you measure the ohms of the fuel injector coils and most are 12ohms give or take an ohm and one is like 7 ohms, you've found your bandit!
When three of my fuel injectors failed last year, one went short almost completely and two others were out of spec.

All you need is a volt meter with fairly long leads and the diagram that I sent. You will have to disconnect the correct connector on the ECM (lives just behind the drivers side front tire on top of the battery, you can't miss it), and then following the diagram measure the resistance between the pin on the connector and the 12v supply fuse which lives in a fuse panel on the side of the dash when you open the passenger door. The hardest part is figuring out which pin is which on the connector in the ECM. The diagram has the pin number on it, you just follow the diagram for each fuel injector down to where the connector is and you'll see the pin number on the diagram. Measure between that and the fuse and you'll have the ohms of the coil on the fuel injector.

Ready? GO!!
Let us know what you find.
I predict a plenum pull.
Don't worry, we'll help you there too!
No need to send out it out for repair, this is something that you can do at home with only a few special tools.
It will help if you're mechanical and aren't too intimidated by the fire breathing monster which lives under the hood of your ZR1

Give a starving man a fish and you've fed him for a day.
Teach him how to fish and you've opened a can of worms.

Welcome to the Brotherhood of the Beast!

:thumbsup:

[tccrab]

I went out to ACS Vette but could not find where they are located. Anyone know?

http://www.acsvette.com/

:thumbsup:

[tomtom72]

Hi Tom & welcome!:handshak: :hello:

Hey guys this don't seem like a typical injector coil shorting issue.....usually once the coils start to short they usually don't go back to not shorting.

IMHO...if you can get the car to do this "miss" while standing still with a scanner on it you might get tipped to where to start looking. Ya know then you can smell the exhaust during the miss to see if it is a lean or rich miss and notice if the miss is only in open loop or closed loop and the scanner info may get a look at any sensor that might be failing....long shots I know. It's just that failing injectors usually don't get good by themselves. Also on that note plugs and plug coils and wires if bad don't fix themselves.....maybe only spark plug wires if ya shake them just right.

JMHO:redface:

:cheers:
Tom

[tccrab]

Hi Tom & welcome!:handshak: :hello:

Hey guys this don't seem like a typical injector coil shorting issue.....usually once the coils start to short they usually don't go back to not shorting.

I<snip>
JMHO:redface:

:cheers:
Tom

Tom:
I agree, injectors dont' usually "get good" for a little while then fail again, but lately I've read more and more acounts of people having issues like this and the injectors were fount to be at fault. That's why I've recommended that he checks them first. The trick will be to check them while the missfire is happening.
It's gonna be either ignition or gas...
It's been my experience that coils or plug wires tend to fail completely, no halfways about it. They either work, or they don't.

[tomtom72]

Wow, that's a new twist to the injector saga! Makes me glad mine just quit so I could find out what the issue was easily.

:cheers:
Tom

[A26B]

.......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.