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'95 started stumbling

reddog

reddog

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My '95 started stumbling last weekend. I figured the fuel filter was dirty. it got progressively worse.
Replaced the fuel filter today and it didn't help so I went through the list.
1) checked codes. There are none.
2) tried known good PMD.
3) checked and cleaned all grounds,
4) changed OPS.
My fuel pressure gauge reads between 4-5 psi @ idle which it has since I installed the gauge six months ago.
When I jumper the relay the lift runs and fills the FFM.
It idles fine, no unusual smoke. It's been running great up till now.
I guess my next step would to replace the lift pump. Does any body have the AC Delco # for the good stock pump?
My gut feeling is it's not the LP. If the FFM was full of fuel at idle wouldn't it run smooth for a while until the FFM ran low on fuel because the LP wasn't keeping up?
The injection pump only has 50k miles on a new standyne.
Any ideas?
 
Burning oil said:
When does it stumble? Under load...ect
Click to expand...
It started out occasionally last weekend under load and has progressed to all the time above idle.
As soon as I start to accelerate it starts to stutter.
Leroy do you sell the stock lift pump?
 
I don't have in stock but can get them $75 shipped.

Sounds like a fuel problem, either lack of fuel are lots of air getting in.
Few things to try:
remove LP & blow out fuel lines with air. Disconnect inlet rubber line at FFM and blow back. I put a rubber hose over the line and into a bucket to see any crap that comes out.
If blowing back to tank fixes the problem it will be a temporary fix and I'd say to remove the tank sock and do a LP pre filter (like I offer of course). I would also clean the tank real good.

Hook up a temp fuel can directly to IP. Use one of those cheap see though filters to protect IP from murrfy's law. If you bleed air from hose you probably wont even need a pump for the test. This will tell you if the problem is/was on the truck or on the IP side of things, injectors return lines...ect
 
So this morning I put in a piece of clear tube. Only a few small air bubbles at first the none at all.
Shut the truck off and the fuel gauge read 6 lb and is very slowly dropping down.
The truck took a little to start. maybe the fuel was bled down? When I removed the return line, to install the clear tube, there was no fuel in it.
Where should I start looking for air leaks? Pretty hard if it doesn't leak at idle or only very slowly.
 
I pulled the supply side off the LP and fuel freely flowed from the tank. No restrictions?
 
Pressure leaks off is normal with engine off.

Run engine, look for bubbles moving with bright flashlight. Stop engine. Look for bubbles in return line. Rev engine to RPM limit and look for bubbles --> RPM (and some fuel demand) will move more fuel through the IP and could cause a restriction to show up. Also watch pressure gauge while revving engine. "0" and a gauge that stops at "0" PSI is useless! "0" could mean a full vacuum actually boiling diesel and causing "vapor lock". Anything above "0" is ok, but you need a pressure/vacuum gauge if you get "0" readings.

Tracking down a kinked hose behind the FFM I would get full flow and PSI out of the water drain. Then when the engine warmed up it would stall out with bubbles in the return line. The bubbles went to full on air with no fuel in the return line. The IP suction would vaporize the diesel when the hose completely kinked.

Have you made sure the OPS is working?
 
Also check that the return line is not kinked or plugging up. Blocking off the return line will cause stumbling and even engine shut down.
 
Just got it to throw code 18 Pump Cam Reference Pulse Error. what does this mean?
 
Stole this from GW on another forum.

"18 and 35 are codes that are consistent with a PMD going south.

Grounds are worthwhile checking for corrosion and tightness.

Is your remote mount PMD harness still grounded to the IP? It should be.

Also try to remove your optical sensor and reinstall it. Corrosion often sneaks it's way in to the connectors and remove/install gets you good contact again. The optic sensor can be part of the "reference error" as the PCM compares the Optical sensor pulse to the crank sensor pulse. If the PCM "sees" something it isn't expecting you get a reference error. It knows something is wrong, just not what.

Air in the lines can also cause a code 18. The optical sensor can't "see" through aerated fuel well, it needs straight fuel. This could be something as simple (and hard to find) as a rust pinhole in a line somewhere.

Low fuel pressure/flow can also cause this. You've replaced the filter and lift pump, but did you check the output? Have you checked the "sock" on the pickup tube in the tank?

Along the same lines with the optical sensor, what fuel are you running? IE: straight diesel, dyed fuel, WVO, WMO, etc. Once again, if the fuel isn't relatively clean and clear, the sensor can have trouble "seeing" through it.

Just for some background, the optical sensor is in the IP. It's the "black thing" on top LH side of the IP with the electrical plug in it. It "looks" through the fuel and there is a disc that spins with the IP rotation in front of the sensor. The disc has slots cut in it and these slots are what the optical sensor "sees" as it looks through the fuel. The PCM uses this input to tailor timing of the injection event. The disc is spinning pretty fast and the slots are a very precise measurement, that's why anything that "impairs" the optical sensor (IE: dye, air bubbles, etc) is a problem.

well, that's quite a few things to start with. I'll stop there for now...."
 
Thanks Leroy, I read that also.
Does the clear tube in the return with out bubbles at idle rule out air in the lines at higher rpm?
I ran a long hose with fuel gauge to the cab. Briefly dropped to 0 at WOT and came right back up but it stutters with 3 psi.
Next step is to pull the upper intake and look at the OS harness.
 
Another note, Ive been buying fuel at Safeway and they started selling B20 a while back. Could this contribute?
Also the fuel out of the FFm and in the clear tube all looks clean and clear.
 
Well I think I got it. Pulled the upper intake for access
to the OS harness. Unplugged the harness, checked for frayed wires and plugged it back in
and ta da it runs smooth again.
That OS harness has at least 3 parts to it. It goes from the OS to a small black rectangular box, then to another black box and then back under the intake
where it can't be seen.
What are the black boxes?
Thanks for all the help. Ill start another thread to talk about my ATT.
 
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