Too much fuel pressure?

[paulj31]

I have recently installed a 2000 long block in my 94 suburban and have put 1500 miles on it. I have been troubleshooting some minor problems since install, replaced damaged crankshaft sensor (have yet to do TDC relearn) new injectors, new vacuum pump, vacuum lines and waste gate sol. I have yet to get this puppy running just right, I get code 35 initially from cold start and it’s intermittent during driving. When cruising under light load trouble light is on, let off fuel and it goes out and does the same with more throttle, seems to eventually go away the longer it runs (10 plus miles) but does sneak back occasionally. FSD or IP? I am waiting for a slightly used FSD and IP from an engine that is going into a mechanical IP. Enough background? I have just replaced the fuel pump relay, ops and the lift pump since it was not starting well. I took it on a trip (50 plus miles) and it ran OK first few miles then starting acting up. The IP was knocking and I had no power on the highway. I check the specs on the new Carter pump and it was rated 10 – 14 lbs! My guess is it was foaming and pumping air into IP. I swapped it out for the old one and it seems to be doing a lot better except now after warm up oil pressure and performance is way down, 30 psi under way and less than 10 at idle. I was running 60 psi and down to 30 at idle prior. It also sounds as if it’s not getting enough fuel. Have I ruined my IP and possibly damaged the lower end running it under these conditions?

[catmandoo]

what about the oil pressure switch as it controls both the gauge and fuel pump,

[catmandoo]

forgot to add (as iv'e seen new ops bad)go back to old and see what pressures like.

[charliepeterson]

I would get hold of a good bi-directional scanner and retime the Injection Pump. These dashboard oil pressure readings can be off.

The lift pump pressure really doesn't need to be as high as your running. I'm pretty sure you only need 5 or 6 psi.

[paulj31]

Thanks for the replies. I am going to install new OPS after work today, timing by weeks end. The engine is definitely running poorly since pump incedent. Does the PCM run engine from OPS on 94's?

[charliepeterson]

Does the PCM run engine from OPS on 94's?

The lift pump is run from the OPS after the truck is running through the ECM.

[paulj31]

I have replaced the ops and so far it seems to be running much better. Thanks for the answer Charlie, so elegantly put. I guess I meant to ask does pcm or ecm use any of the ops data/circuit to control engine. It was running so bad prior to ops replacement I thought the IP was toast from the high lift pump pressure mishap. I want this thing to run as well as it can stock before I start modifications.

[ucdavis]

On a 94 there is nothing the pcm "reads" from the OPS that goes into running the engine. The sending unit runs the oil gauge, and the pressure switch (OPS has these two functions) runs the lift pump after oil pressure is about 5psi or more. If the switch part dies, which is fairly common, the LP gets insufficient voltage to send fuel to the IP which starves on heavy acceleration, but there is no function of that in PCM operation (no codes, no changes to demanded operating parameters from PCM to engine).

[paulj31]

I appreciate the feedback but I think the main point is being missed. I bought the lift pump from a national dealer who has four initials and two of them are

[charliepeterson]

They are selling these pumps as replacement for original; which have far too high psi for the application. Could this have damaged the injector pump?


Good question.

These Injection Pumps really need a good supply of fuel for many reasons but how much is too much?
I doubt you did any damage to the pump. How does the fuel filter look after running with this extra pressure? Do you still have the small filter on top of the fuel down pipe inside the filter housing? I bet 99% of us don't. This is supposed to be the last line of defense for dirt going to the Injection Pump.

[Turbine Doc]

I was in my 98 GM manual looking for something else and came across this paragraph describing lift pump operation " the lift pump is used to deliver fuel at a low pressure (at least 3 psi or 21 Kpa) at rate of .24 litre (1/2 pint) in 15 seconds (15 gph)."

The lift pump is controlled by the PCM (in 98). (PCM control is later models, OPS pump control is early model controller as others have stated; pre 96 if I'm correct)

The 98 book as I read on also in next paragraph reads " when key is turned on PCM energizes fuel lift pump relay, the lift pump remains during glow cycle. If no start immediately following glow complete, PCM shuts off fuel lift relay, and waits until engine rpm is above 0, then turns relay back on. As a back up system lift pump can also be turned on by OPS. When above 4 psi or 28 kPa oil press, and the fuel pump relay does not complete the circuit, the fuel pump/OPS will close and complete the circuit to run the fuel lift pump."

[ 10-28-2003, 10:57 AM: Message edited by: tbogemirep ]

[paulj31]

Well I had my buddy come over with his Tech 1 tonight to do the TDC relearn. He checked out my codes 35 and 36, scanner read 2.09 on pulse width and as high as 2.22. Injector pump going south from high fuel psi/flow mishap? I have a few symptoms of the FSD going such as surging in reverse. The timing is -.09, not good but will wait until I get hold of another FSD and hopefully rule out the IP for a few more miles. The dealer who sold me the lift pump and faulty OPS refunded my money, didn

[ucdavis]

DTC 36 w/pulse widths up to 2.22 sounds like IP going out, but I'd try a known good FSD first just to be sure (R&Ring the IP is not my idea of a good time, but that's just me). It could be signal-to-FSD-for-std.-pulse-width but FSD clowning up the signal to fuel solenoid. W/the surging (which has to be fuel related) the FSD is certainly suspect.
The pulse width window is narrower on the early electronic models, FYI; I read up to 2.02 routinely on mine. Don't know the stock threshold for tripping a code 36, but it must be above 2.02. Later models give more latitude B4 tripping a code. Don't know if this could be programmed out to later model specs in the chip; anybody else?