Stumbles on acceleration
Thread Starter
Intermediate
Joined: Feb 2008
Posts: 46
Likes: 0
Stumbles on acceleration
Ok stock 86 tpi. No codes I checked. It runs rough and rpm goes up and down just alittle at idol when cold. If you hit the gas it wants to die, then will rev up. When warm it idols ok but when you stab the gas in park it stumbles, then revs up. I have put plugs, wires, cap, fuel filter, fuel pump and sock. Tps is set right .54v, changed MAF with another and that didn't help. Timing is set at 6 with the est disconnected. I am getting 16 inhg vacuum at the right rear of the plenum. I found 1 vacuum leak and that made it run alittle better but its still no where right. Any ideas oh corvette masters...
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
Most will agree that the relays should be changed when the MAF is changed...maybe you should try that, I think they are ~$15 each. They are located on the firewall behind the battery.
Don't know if that'll help you out, but its worth a shot, especially since you were supposed to change 'em out anyways.
Also, have you checked the fuel pressure?
Don't know if that'll help you out, but its worth a shot, especially since you were supposed to change 'em out anyways.
Also, have you checked the fuel pressure?
Thread Starter
Intermediate
Joined: Feb 2008
Posts: 46
Likes: 0
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
fuel pressure looks good; did you leave the gauge on the car for a while after you shut it down. It should hold pressure for a while.
Miles, I've been goin' through the same thing as you for 5 months. I think I've finally tracked it down to a bad maf burnoff module ('85s only). It does the same thing as the two relays.
If you have a volt meter or multi meter, you could check the function of the relays before replacing.
Miles, I've been goin' through the same thing as you for 5 months. I think I've finally tracked it down to a bad maf burnoff module ('85s only). It does the same thing as the two relays.
If you have a volt meter or multi meter, you could check the function of the relays before replacing.
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
Miles, the relays on the 87-89 are the same, but there is a difference between the two on your '86. I couldn't tell you anything about the autozone ones, I buy most of my stuff from Mid america. Here's a link to the relays:
http://www.mamotorworks.com/corvette-3-139-6128.html
the ones from autozone would probably work, you'd just have to make sure which on is which.
http://www.autozone.com/N,15901319//...eResultSet.htm
http://www.mamotorworks.com/corvette-3-139-6128.html
the ones from autozone would probably work, you'd just have to make sure which on is which.
http://www.autozone.com/N,15901319//...eResultSet.htm
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Burning Brakes
Joined: Jun 2005
Posts: 1,188
Likes: 13
From: Wallingford Vermont
IIRC the original relays carried different part numbers, the newer style is the same for both. I bought one years ago when my burn-off relay crapped out. 'Got the new style from the dealer. All is good for a day or so then I got a different malfunction code 
Back to the dealer, I tell the older parts guy who grins "they didn't read the service bulliten, both power and burn-off have to be replaced together when using the new common relay"
If you search you should find the numbers and such...
Back to the dealer, I tell the older parts guy who grins "they didn't read the service bulliten, both power and burn-off have to be replaced together when using the new common relay"If you search you should find the numbers and such...
so now I am going to dig into the injectors maybe they are leaking.
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
Hmm...that seems on the high side. To my understanding, it should be 12v...lemme do some searchin'...
couldn't find anything...maybe someone else will jump in on it. I would think that 14v is too high, meaning a bad power relay, but I really don't know.
try this, pierce the C&A wires at the MAF and get a volt reading between the two with the engine running and MAF connected. Should be between 4-6V. If it sends too much voltage, it might return too much also, making the supposed to be 4-6 be ~8...who knows!
couldn't find anything...maybe someone else will jump in on it. I would think that 14v is too high, meaning a bad power relay, but I really don't know.
try this, pierce the C&A wires at the MAF and get a volt reading between the two with the engine running and MAF connected. Should be between 4-6V. If it sends too much voltage, it might return too much also, making the supposed to be 4-6 be ~8...who knows!
Last edited by cwyates4; Aug 14, 2008 at 11:51 PM.
Thread Starter
Intermediate
Joined: Feb 2008
Posts: 46
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ok did some more tests. fuel pressure gauge hooked up, car running its at 39, went around the block under load at WOT goes to up to around 42 so thats all good. shut it off and pressure went to 20 in less then 2 minutes. humm, I went back under the gas cap area and pinched off the return line. Turned the key on, built up alot of pressure and turned the key off and the pressure fell to 20 in under 2 minutes again. So I'm thinking leaking injector? next is to remove the injectors from the base and pressurise the rail and see whats going on right? or I will wait till it cools down crank it over a few times. Then start pulling plugs to see if they are wet...
Melting Slicks
Joined: Jan 2008
Posts: 2,065
Likes: 1
From: Jasper Texas
If you pinch the return line and still lose pressure (assuming its a solid pinch) you have leaky injectors, more than likely. You can do either test you'd like, but if you do the first one you mentioned, make sure to have some rags handy and use a pretty deep bowl or somethin' to collect the fuel.
Mine dropped to 0 in twenty minutes when I had 2 leaky injectors. Now, it holds 20 for over 2 hours and holds pressure over night.
Mine dropped to 0 in twenty minutes when I had 2 leaky injectors. Now, it holds 20 for over 2 hours and holds pressure over night.
