OK, so I have had this problem with the engine feeling like it's dropping 2 or more cylinders and spluttering and stumbling, when warm and pulling away from a stop. If I put it in neutral (T56), it will rev fine, as soon as I put it in gear it stumbles again.
So far, I have done:
New O2 sensors
New Opti
New H2O pump
New intake gaskets (fixed the rear leak)
New coil (oem)
New Coil Module
New plugs and wires

I am running full length headers, no cats, no egr equipment (disabled physically and Programming)
There are no vacuum leaks that I can find---searched extensively.
I have no codes...at least no light....

Please help with suggestions!! I'm getting a bit disheartened...

BTW...this is a 1994 Z28 drivetrain in my 240Z!

Thanks,
Tim

 

Since you pretty much replaced all of the ignition components that could be the culprit, I'm going to guess fuel system....check fuel pressure. Common things to go are the fuel pump, fuel filter, and fuel pressure regulator.

 

I have 34lbs at idle. I'm using an MSD inline pump (fuel cell) and -8 lines. When I unplug the regulator vac. line, it jumps to 47lbs.
I might try a Walbro pump, but I'm getting a bit fed up with buying parts unnecessarily.....
I have almost a full cell of fuel.....

Tim

 

Have you checked the injectors or had them checked. I had 3 bad injectors before and it would make the car run like it had dead cylinders especially at idle with a load on it.

 

The injectors are my next stop.
When I first fired the engine (It had been sitting for a couple of years) a couple of the injectors weren't firing (stuck from sitting I would think). Well a couple of short taps on the injectors with the back of a wrench freed them up and all was OK.

My next thing I will do is get some injector cleaner to attach to the rail and run that through. Failing that I will take the rail off and take the injectors in to get tested.

Thanks guys....keep em coming...

Tim

 

yea, I think you have found your problem. You most definitely have a fuel system problem and since your injectors have given you a problem in the past and are old and have sat for a while, well... You can have great fuel pressure and bad injectors obviously. So get them checked by a shop for flow and then go from there...

 

OK, I decided to just get new injectors. I didn't really see the point in having the existing ones checked, and figured putting the $$ used for the injector check would be more wisely spent on just replacing them, so I bought a set od Accel 30lb injectors. I also began looking into the fuel pump. I am running the MSD (2225) inline pump. Looking at the MSD spec sheet, the pump delivers 43lbs/hr at 12 volts (~165Lph) and about 190Lph at 13.5 volts (extrapolating from the curve chart). It seems that at maximum output, that pump is barely providing the minimum requirements. I doubt that this is causing the low speed stumble that I am experiencing, but the pump is EXTREMELY noisy, so I am also replacing it. I have ordered the Walbro 255 LpH inline pump.

I will report back upon changing the injectors (I will wait to change the pump so that I can confirm that the injectors were the problem), and the pump just to close the loop on this obviously common LT1 issue!!



Tim

 

You're gonna need computer tuning in order for those injectors to work correctly.

 

No problem, it's a simple input with TunerCat

Tim

 

I thought stock fuel pressure was 43 PSI. Are you running a adjustable fuel press regulator? If your pressure is low you caould be expreiencing a lean condition that could cause it run run rough or miss.

All inline fuel pumps are noisy, it's part of the reason all OEM manufactures put them in the tank.

 

I tested the pressure on my bone stock Z28 and got the same idle pressure. Although the Factory Service manual is not specific as to what the pressure should be, it specifies that it should be lower than 45psi.
I am running an oem regulator.

I appreciate that the pump will be noisy, but when I can hear the pump over the freeflowing 3" exhaust, that's a bit rediculous!!
I have it mounted with the supplied rubber covered clamps to a sheet of 1/8" thick aluminum plate which is then mounted via rubber isolators to the chassis. It used to be much more quiet, but seems to be getting louder and louder as time goes by.

Tim

 

43 is stock pressure with the vacuum line off. Normal idle pressure with will be considerably lower, due to less demand for fuel. That's the whole purpose of the regulator.

 

When I disconnect the vacuum line to the regulator, it jumps to 47-48 psi......that's still ok, right?

 

yes, thats fine. Its when its under at any point that you should worry. Or when its ridiculously high