Dissolve

'91 LB9 5spd. Paxton SN92 supercharger running about 7lbs of boost. Stock internals. Usual bolt ons (full exhaust, ported polished intake, crane 1.6 full rollers etc etc etc)

Was running great until earlier this year. Pining in the upper RPMs during WOT now. Fuel pressure (AFPR) is at 42lbs@idle on 24#svo injectors.

Any ideas? Should I disable knock sensor with a resistor?
Thanks

TheGreatJ

Who did your tuning?

If it's only at higher RPM, under load, I'd say it sounds like fuel starvation. Tape your fuel pressure gauge to the windshield and see what it reads when you get the ping. Idle pressure looks good, but the pump's volume may be down. If it's a little low when you get the ping, it may just be a dirty fuel filter. If it's badly low, then it's tank-pullin' time.

Don't disable your knock sensor. It's what tells the computer to retard the timing to keep you from blowing holes in your pistons.

Dissolve

Fuel pressure gauge shows steady increase in pressure as throttle increases. I have a FP gauge in cockpit. Never pinged before on 42#s of fuel pressure. That is the baffling thing. Ran 42#s in all temperature conditions for years, no problems.....until now

I used a resistor to disable my knock sensor before. You need to sometimes because of all the damn drivetrain noise my 1.6 rollers make. I put it back on when I went blown

89Warbird

IF you truly are not having a fuel supply problem you probably have some carbon buildup on top of your pistons due to low quality gas. That means even high octane no name gas. You will have to get it out, many differnt ways to do it. I'd try loading up the tank with Techron first, if that doesn't work some water or tranny fluid into the throttle blades will knock it out. Though I've never tried it on a FI car, just carbs.

Bad on me, check the total spark advance as well. You'll need a dial back timing light to do it.

Fuel first, timing second, carbon third. I'm also assuming it's not getting too hot. What kind of plugs? Did you try going colder?

Dissolve

Mostly used 94 oct Sunoco. I will look further into this though.

Timing is at stock as per Ed Wright's recommendation to run with his FastChip

Plugs are AC DELCOs running 2 stages colder

89Warbird

You still need to check the total timing advance and that can only be done with a dial back timing light. Did this start with a new chip?

Dissolve

Last time I checked timing, it was stock (8* BTDC I think. Or whatever stock is)

But I will check it again ....car has 130k on it.

DuronClocker

iTrader: (1)

Stock timing is 6*BTDC.

Still as said above you NEED to get the total advance at WOT to see if it is advancing properly. Base timing is great to have in spec, but if your car idles fine and only pings in the top, then you have an issue at WOT, not idle.

With 130k, carbon deposits could very well be the culprit, even with high-quality gas. Use a spray bottle to spray some water into the throttle body while the car is running until you see black crap dripping out of the exhaust tips. That's the carbon getting blown out of the cyls.

tom3

Valve springs?

DuronClocker

iTrader: (1)

Could cause valvefloat but not pinging...

89Warbird

Tom3 may be on to something, what RPM is it occuring at? Does it fall flat on it's face?

Also, all advance should be in by 3000 rpm, but you don't have to be at WOT to check it, just run the RPM up and see what you get.

doc

If nothing has been changed on the car or engine, then the root cause would be something that changes with time, like carbon build-up and weak valve springs. Also, spark plugs can change with time, as well as fuel injectors. Concentrate on these issues.

Pull the plugs and check them, do a compression check with leak down. run top end carbon cleaner, use fuel injector cleaner,etc.

How about the fuel filter?

Dissolve

Thanks for the great suggestions. Been so busy at work havent been able to check the board.

The blower redline is before 5k. So I always shift like around 4800 which is well before redline. Car has stock cam but does have 1.6 full rollers. Going to try a few of the other suggestions.

thinslice

How are you suppose to spray water into the plenum if the thing will not idle with the MAF not hooked up?

DuronClocker

iTrader: (1)

Mine will idle at least a little bit. Disconnect the electrical sensor and it should run in limp-home. If it wants to die, give it a little bit of throttle (just a little) and spray the water in.

robsgta

Are running an inline pump as well as the factory pump..
When your chip was burned was it burned with the supercharger spec'd into the advance equation.