MIZR1

Last summer my 91 ZR-1 began to exhibit a minor stumble on acceleration. I took it to the local Chevy dealer who did a diagnostic and told me that the cause was a fuel pressure regulator and the damage would be over $700 for a new regulator and labor.

I balked and have driven it since, but the problem has worsened and I cannot ignore it. On hard acceleration after it warms up it stumbles quite noticeably and there is even a stumble at idle periodically after it is warm.

I have replaced the fuel filter, and I replaced the fuel pumps a couple of years ago. The car has almost 40 k miles.

My question is: does anyone on the forum have experience with these symptoms and is it likely that the dealer's prognosis that it is the fuel pressure regulator is correct? After researcing this on the web, I am not so sure, so I thought I'd ask the experts.

I will greatly appreciate any input you can provide. I have the Service Manual and between that and the plenum removal instructions on the web I believe I could do the job. But I would appreciate any pointers.

Thanks,

Glenn

8388

If you want to rule out the fuel regulator, rig a fuel pressure gage so that you can see it on the windshield as you drive. Then if you notice the stumble at the same time you lose or gain pressure then you can go to the next step. Regulator or fuel pump. I would first make sure you don't have a vac leak. Especially at the map sensor.

90 Corvette ZR-1

Does it only do it when the car is warm?

MIZR1

During the first 10 or 15 minutes after I start it up, I don't notice any hesitation, and it seems to function at full power. After it warms up, the hesitation begins. I haven't yet tried to measure the vacuum. I am really looking for some confirmation whether the dealer's idea is likely the correct one.

Unfortunately, I don't have a fuel pressure gauge, although I am now shopping for one on the web. Mityvac has a vacuum/pressure gauge for about $75. It is part no 4050. Would this be a suitable tester for a fuel injected car?

I have been having some frustration finding a diagram of where the MAP sensor is located on the motor, via the Helms manual. I assume it is towards the back of the engine.

bad-zr1

The sensor is located at the very back of the plenum. You can just barely squeeze your hand in there (from the drivers side) and check if the hose that attaches to the plenum is hooked up. The other thing to check is when the car is first started you run on both fuel pumps untill the temp gets to 176 or so and then one pump shuts down. The one that is left running may be going bad on you.

jabrun

My 91 had similar symptoms a couple years ago and it ended up being one of the wires going to the o2 sensor on the left bank. I fought the problem for a long time and it didn't throw a code until it got really bad. I think if you turn the key on for a while with the engine cold you can put your hand on the sensors and should feel them warming up. Once I had a bad plug wire that gave a rough idle and hesitation also.

90 Corvette ZR-1

Sounds like it could be an injector problem.

A26B

The Mityvac combination pressure/vacuum gauge you are looking at does not have a fuel bleed system and the fuel rail hookup hose/fitting. You really need two seperate instruments to perform these tasks. If you're going to get into doing your own work (I recommend you do if you're going to be into it for the long haul) you'll need to perform these tests in the future to properly diagnose problems.

Vacuum Tester
http://www.actron.com/product_detail.php?pid=16292

Fuel Pressure Tester
http://www.actron.com/product_detail.php?pid=16174

These are typically available at Autozone.

Eventually, you will also need a multi-meter to work on the electrical components.

By using the search tool on the ZR-1 forum, you can find a lot of discussion on checking the fuel pumps and the fuel regulator.

Good Luck.
Jerry

kabnvtt

Is the stumble constant or is it just one stumble as you accelerate? When I changed my plugs I set the gap to wide and it had a stumble as you have explained. Have you checked the plugs. Just a thought, good luck.

MIZR1

Jerry, thanks very much for the response. That is exactly the kind of direction that is very helpful to me. I don't mind spending a little extra on the tools as long as I am getting what I need to do the job. And, I totally agree with you that I need to learn how to do the work myself rather than relying on the dealer, since I plan to keep the car until they pry it from my cold dead fingers. Once again, thanks for the thoughtful reply.

Kabnvtt, I was thinking about the plugs as a possibility, but the stumbling seems to occur only under relatively hard acceleration, and it continues until I back out of it a little. When I spool her up gently the symptom doesn't occur. However, with nearly 40 K miles, it is probably time for a plug change anyhow. Thanks

ZR1 MK

Possibilities based on your description.
Fuel Pump (If the car runs good until 176 degree coolant temp, your primary fuel pump may be at fault. Check pressures for each pump at at idle and under load/rpm).
Fuel Preesure Regulator (If fuel pressure is ok, you know the reg is ok. If the fuel pressure if out of spec, you know its fuel pump or reg [as the dealer mentioned]. Check as indicated in the manual).
Vacuum Pump (Check pump and lines for vacuum leak).
Fuel Injector (Injectors can be checked electrically and mechanically. I have my org injectors except one. It passed check electrically cold and hot, but failed mechanically).

Mylzi

I had broken fuel regulator on -90 ZR1 couple years ago.

It did leak fuel to plenium from vacum line.

Problem was that it was difficult start car, course when you did turn key it started leak fuel to plenium.

But I dont think that new regulator was very expensive part.

JordiZR1

My bet goes to
1. plugs
2. wires

ZR1 MK

Based on supplied info, I figure the the wires and plugs would be ok. If they are suspect, check wires resistance and covering for damage. Check performace of each cylinder for plug issue.
Keep in mind, when it comes to plugs and wire, most people replace both unnecessarily.

steve carrington

If You Haven't Changed Your Ignition Coils I'd Say That Could Be Your Problem. My Z Did The Same Thing Yours Is. I Changed Them The Car Ran Perfect Again.

steve carrington

If You Haven't Changed Your Ignition Coils I'd Say That Could Be Your Problem. My Z Did The Same Thing Yours Is At 30000 Miles. I Changed Them The Car Ran Perfect Again.

secondchance

I had my fuel regulator replaced due to leaking and before the replacement did not exhbit symtoms you are describing. The part was'nt that expensive.
Call and ask Bill jr. at P & G Chevrolet @ 888 870 0280 X 214 for regulator price quote.

Jeff Begor

I have a 90 experiencing the "exact" same symptoms. From many discussions I've had with a number of qualified people , Marc Haibeck being one of them my guess I'm going to find one of the following to be the problem.

Vacuum Leak

Fuel Pump and or Regulator

Or possibly a shorted coil on one of the injectors according to Marc
Haibeck. Once the car is warm shorted coils have problems more than when the engnine is still cold. He also says that a shorted coil can cause the hesitation we experience.
When the weather gets better here I'm going to take it over to the Corvette Shop where I bought my Z.. If anyone can fix it Paul can..

1990 QUASAR BLUE

This is a nice little gauge, I bought the same thing from Sears for about 45.00. The only problem with it is with the Z, I needed to remove the IAC to get it on without pinching it a little and skewing the results. Also the hose is a little too short to tape it to the windshield for a road test. http://www.madvet.com/shop?frame=1.13.172 This one has a longer thinner hose and an angled valve which makes the aforementioned much easier on the Z.

MIZR1

All, thanks very much for all of the good leads. As soon as it warms up a little in my garage, I plan to run them down and expect one of these leads will solve the problem. 1990 QB, thanks for the tip on the pressure tester. I havent yet purchased one and need to.

Regards,

Glenn