Getting some backfire
 

I recently put a new fuel pump in my 85 vette which solved a really bad hessitation i was getting. But my problem now is that every now and again i get a horrible poping and a total loss of power when i first start driveing the car. It usually only happens when i first get off work and begin to drive home. I let the car idle for a while before i begin to drive. (usually let it heat up to about 150).

Im a new vette owner but my dad has had them for years. I actually helped him rebuild the motor back a few years ago. It has been modded and I dont honestly remember all of what was done. Any thoughts on the matter would be great. Thanks in advance.

welcome to the world of Corvette!

Got to know whats in it....carbed? still TPI?

fuel filter done recently? fuel pressure? MAF?

all kinds of info required for even a half ass guess..
start with the tpi or carb, then IF its tpi lets look at fuel pressure.

Thank you for the reply leesvet. I am to check the timing and fuel presure tomorrow. Sorry for the lack of info im honestly used to working on a 1980 mazda rx7. Little new to big ass motor thing lol.

You said its been modded...how so?

is it still fuel injected or is it a carb now? The next steps depend on the answer to those questions...
regardless, not even sure of what the question is, someone here can answer it !

You're in the right place for help.

Its still fuel injected. I honestly dont remember all of the mods it has but not much of it is stock after the rebuild. Im going to get the fuel pressure tomorrow. I also talked to my dad (the former owner) and he thinks its going to be a timing related issue.

Ok I have done some tinkering this weekend and i have my timing set to a 12 advance at the moment. I am getting about 34-40 psi on the fuel rail. when I have the key in the on possition i have no presure. It also bleeds off as soon as i turn the car off I thought that a fuel injected system was supposed to hold pressure even if the car is not on.

I am honestly not sure how to get a maf reading. thats the only thing i saw that i couldnt get an answer for. Thanks again for the help so far.

My misfire popping this past summer was due to ignition components. Check them all. If mileage on them is high I would replace. My cap-rotor lasted 50K miles and was seriously worn.

Thanks for that rohn I didnt even think about looking at it. Supprisingly I actually have a spare dist cap. Im getting ready to put on some new spark wires. Do you think its possible to that the coil is bad? how would i go about checking that. It has gotten progressivly worse and as I stated before im really not used to the v8. thanks for all the help so far.

Pretty sure it should hold pressure. May be pressure regulator gone bad. 12 degrees advanced? Seem too high but did you time with that spark control wire disconedted? It has to be. Get a factory service manual CD from eBay for $10. It will be the best $10 you will spend.

When I got my hands on it it had a 14 advance on it. Do you think i should drop it around 10? and how would I know if the pressure regulator went bad?

I think factory may be 6 degrees. You need to find the sticker under there somewhere to tell you the advance. But on my 88 you have to disconnect the spark control wire near the brake booster before you set the timing or it will be wrong. A bad pressure regulator will allow the pressure to bleed off after the key is on but not starting the car. Could also be the output line on the pump not tight allowing air into the line, but I guess then gas would leak out the back of the car. Didn't you say the pump didn't run when the key was on but not started?

I did disconet the spark control wire. Theres not much left stock on it which is why i have the advance so far. I recently replaced the fuel pump. It ran better for a while and fell right back into the same stuff within a few days. The fuel presure with they just in the on position without the motor running is at 0. After i turn it on it stays at a fairly steady 36. When i shut it off the pressure bleeds off quickly. I can defenetaly check the hose on the fuel pump. very possible I left it a little loose it was about 10:30 pm when I got the time to change it.

I would check firing order to make sure no wires crossed. If fuel delivery is stock, you should have pressure with key on. May be fuel pump relay. 36 lbs stock is too low also, regulator may be bad. There are procedures to check all this stuff in the service manual.

Fuel Pressure
Key on 45 running 38 pull vac off 48 accelerating 55

Bleed down 20 minutes 42 to 41 PSI
These are my readings on fuel pressure. Stock 89

Some things to think about:

Find out where fuel is returning to tank. Could be from the FPR. Or the check valve in the new fuel pump. But if it bleeds down right away as you said, that should not be too hard to find and should be repaired as a starting point. Check the return lines going back to the tank.

Fuel filer, probably not bad but when was it replaced last?

Debris in tank - might cause a problem

Does the pump run when the key is turned ON for 2 seconds?

Drive with the fuel pressure gauge attached so you and monitor it.

Check vacuum line of FPR to make sure it is dry and there is no fuel in it.

85s have had a problem with fuel injectors leaking. Might be part of one of your running problems

In reviewing the diagram I have, the 85 only has a relay for the fuel pump and not an additional power switch from the oil pressure on the later cars do. So any problem with the relay, you could lose fuel pressure.

Thanks again all Im going to be doing alot of this stuff tonite so I will get back on tomorrow and let you guys know what if find. This forum is awsome Ive never gotten so much help so fast on a forum. thanks again.