engine power trouble

[Terry Callihan]

I recently purchased my zr-1. I took it to a chevy garage to find out why it is hesitating when you get on it.The people at the garage told me that the car has a bad vacume pump. [after a week of haing the car] I have since then voined the registery hoping to get some in put from people who know about these cars.the car only has 9300 miles on it, and i'm not so sure that is the problem.If anyone can give me some input I would be greatful.

[1990 quasar blue]

If you turn the key on without starting the engine you should hear the pump run briefly. It is located where the passenger side headlight would sit with the hood closed. Also, is the check engine light on?
http://www.corvetteactioncenter.com/forums/showthread.php?threadid=14272 This link will help.

[tomtom72]

You may, if you're not already a member, consider joining the Registry. I say that because over at our ZR1NetRegistry web site the latest newsletter, "Heart of the Beast", has a Tech write up by Marc Haibeck explaining the operation of the secondary port throttle system mechanically and from a vacuum requirement view point. I found the article to be most revealing and easy to follow. You could do your own trouble shooting of the system from the info in Marc's article. Kudos to DDSLT5 and Marc (the man) for a great newsletter!:thumbsup:

Anyway, if you turn the key to "on" and go listen by the pump you should hear it cycle to charge up the system and then every so often it may re-cycle for a couple of seconds. If you don't hear it at all then unplug the vac line that goes from the pump to the hose that goes under the plenum. Over at the right water tube @ the I/H near the alternator you will find that coupler. If you unplug at that spot and the key is on the pump should run & if you cover the end from the pump the pump should stop. You can also put your mighty vac pump there to test some of what is under the plenum...not all of it but if the vac gauge drops like a rock then you may have a problem with: Actuators, or control solenoid, or check valve, or the Tank, or any of the connection points in the secondary vacuum spider. To do this basic preliminary test you only need to put about 10" of vac on the line going under the plenum & see what happens. If when testing that line, you get it to hold vacuum then the solenoil will need to be activated to then test the actuators. You can see them thru the gaps in the runners with a flashlight. That's what learned from Marc's article. I can go re-read it and give you the proceedure if you need it.

:cheers:
Tom

[Terry Callihan]

Guy's,
I appreciate the info,the first thing i'm gonna do is go get my car.It took them a week to tell me what they did.

[GrayZ]

another area to check is easy
your map sensor hose is located at the center rear of the engine
under that aluminum bracket..if the hose is soft and pliable..that
could be your culprit..my car had an issue and that was the problem

[Tom]

A non-operational secondary vacuum pump will not cause a hesitation. Once the engine is started the secondary vacuum pump only operates if you stay at full throttle for a long period, longer than most people will have room for. Normal engine vacuum operates the secondaries most of the time. I would recommend purchasing a shop manual since it explains how the different systems work and provides schematics for the systems. Most of the stuff is standard Corvette but there are some ZR-1 specific items such as the secondaries that you need the manual to understand.

You might just be feeling the normal opening of the secondares. On my 90 you can feel the secondaries open under certain throttle openings and shift rpm. Some people might call the way this feels as a hesitation. Different years used different programming for the secondary throttle opening. I have seen an article about the programming used in different years but I forget if it was on the website, Marc Haibeck's website, or in the old ZR-1 registry magazine. Try driving the car with the same throttle openings and shift rpm with the full power on and off to see if you are just feeling the secondares opening.

Also, it would help people possibly diagnose your problem if you could give the year of your car and as many details as possible about the problem, such as throttle opening (wide open, 1/2 open, etc), rpm when you open the throttle, shift rpm, gear, etc.

[Jeffvette]

Terry, several things could be causing your hesitation or stumble. I would recomend Marc Haibeck as the closest guy to contact. www.zr1specialist.com

But possible things to check, Injectors, coils, cracked plug or fouled plugs and plug wires.