High rpm Missfire
 

85 vette, I'm having misfires on the top end between 4000-5500 rpm. Pulled the spark plug and they are showing lean. Tips are white. Running at about 27-30 psi fuel pressure.

Any thought? I haven't been running it at all since I pulled the plugs and saw the lean condition. Injectors? Any sensors that could be bad?

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Car has about 19,500 miles. Sat mostof it's life. Purchased it from my father recently.

Pull the vacuum hose off the fuel pressure regulator. Is there gas in the hose?

When was the last time the fuel filter was changed?

Keep in mind that it's also going to be normal for your engine to cut out between 5000 - 5500 RPMs because of your intake design. The design of the intake chokes the motor from going any higher.

Your fuel pressure does look low, and your plugs do look lean.

If you can pinch off the return line hose in the back of the car, under the gas cap door rubber insulation, and prime the fuel system, does the pressure rise? Or is it still kinda low?

No gas in the hose that I saw. Not sure when the fuel filter was changed last.
It's missing like a rev limiter. Just cuts it like that. It was happening around 5000 rpm and then last time I drive it, it happened at 4000 rpm. That's when I pulled the plugs and found the lean tips.

What is the fuel pressure supposed to be. Seems like everyone on this forum is telling me different pressures. I've been told it should be 25psi some people say 40psi. Hard to get answers sometimes.

Also, I just got back in from movie the car out of the way. When I got out after parking it, I could smell some gas in the exhaust fumes. Could almost taste the gas in the fumes. And the other day when I had it out before I parked it my father was behind me on his motorcycle and said he could smell the gas from the exhaust also.

Well if that was the case, your plugs would show it. They would be black, or dark.

What is your timing set at with the EST wire disconnected?

for the miles on the plugs they look fine. What does your fuel look like I would look there that low milleage and you say it was sitting I would get the fuel out and replace it.

Im not sure about an 85 but my quess is you are hitting real close to the rev limiter, if it has one. Not sure what year they started but that is something you need to check

I ran out the tank to almost empty and have filled it completely back up. Didn't run any different between the old gas and the fresh gas. I've put about 250 miles on it so far.

I also thought it was the limiter and I've tried to search and ask about what the limiter is and I get different answers from everyone in here. A lot of people say there is no limiter but the intake design restricts the rpms to 6000-6500. I'm not too sure. But if it was that, it wouldn't make sense of how it would hit that miss at 4000 one time and then other times 5000-5500.

Can anyone tell me what the fuel pressure is actually supposed to be?

40 to 43 psi idling with the regulator vacuum line disconnected. Should also be at that reading at wot.

Okay, now with the vacuum line disconnected, should the vacuum line be let open or plugged, I'm speaking of the actual line you take off?

Also, what is normal idling psi with the vacuum attached? I did a search in google and good a few results that say 35-39psi is normal. I am going to retest the fuel pressure again after work today. So I can post the results.

Plugging the line will not effect the fuel pressure, but I would recommend plugging it so the idle is easier to control. The pressure with the line connected will be between 30 to 38 PSI depending on the engine vacuum. The higher the vacuum the lower the fuel pressure.

Johnaitor dod you have the org owners manual? If you look in there under tach it should say. I know 88 cuts off the injectors at 5400. I know this even though the intake is poor. You will not get a miss from it infact I would doubt that you would feel it running out of power in the lower gears. Sure the dyno would pick it up but you wont. Ill do a little more research and see what I can come up with

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of course!

If you are not maintaining a steady 38-40 PSI at ANY rpm, its going to be lean and misfire under load or at higher rpm. Period.

There is NO rev limiter in TPI. Especially in an iron head 85.

Misfiring at rpm in these engines is caused by one of 2 things.....

low voltage at the spark plug (or gap too wide)

or low fuel pressure.

I'm surprised it even runs at 27 psi. Mine won't. Takes 30 to build a fire in these things..

You are so wrong on the rev limitor. I do not know what yera you have but I know for a fact that at least 88 do. Look in your owners manual under tach see what it says I just check to be surebut my says. Fro added engine protection a high fuek shut off has been incorperated into your corvette. When the engine exceeds 5500 RPM the fuel injectors cease to function.

Im am reasonability sure this was true for all injected cars in the gm line up starting in 1985. I have a fleet of calivers and the rev limiters were set at 4800.

Did another fuel pressure test with someone helping. Was getting. 35psi at idle. When I got to roughly say 1500 rpm is dropped down to 20 and was on its way slowly down further to more rpms I gave it.

try checking like this key on not running pressure should be 40-47,if that is ok at idel pressure should be soms where between 37 an 45 then apply 10 in vacumne and the cprevious numbers should drop 3 to 10 psi this is the correct way to test a reg

I am not sure who told you this but there is no factory rev limiter on a TPI engine. I have tuned many TPI cars and never seen a rev limiter set unless you consider 10,031 a rev limiter, because that is what it is set to. Essentially no rev limiter since the engine will come apart well before 10,031 RPM. (expecially a 1988)

That is your problem, fix the fuel pressure and the car should run fine.