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Discussion Starter #1
Let me say first I am thankful for this site. I've been in lurk mode as I haven't had much to offer. Now I am finding I need help, so here it goes...

I bought my Yukon with the intent on running SVO/WVO. It's been running fine on both fuels since September '06. I did notice what sounds like a gasser pre-ignition ping. No DTC's.

When it turned cold in December, it started stumbling, missing, and losing power right after a hard acceleration or long incline while on diesel. It also had long crank times to start. Switching to VO made it run like a champ. I have a see-thru in-line filter on the return line out of the IP and noticed very little fuel coming out of the IP in diesel mode. Still, no DTC's.

It got to the point where it would completely stall and I'd have to re-crank it to get it run and could only operate at 45MPH. Again no DTC's.

Reading some on this site I determined that given its age and thinking it was a fuel delivery problem, I replaced the lift pump, OPS, and filter this past weekend. I also checked it's operation, check for air leaks along the route, and the fuel flow to the IP and from the fuel manager (I wanted to make sure the extra valves from the WVO conversion wasn't a factor).

While trying to restart I saw no fuel from the IP return line.

Using the shop manual, I tried running through a no-start diagnostic. Disconnected the optical sensor and got nothing. Cracked one of the injector lines and saw no fuel.

Then it the manual got beyond the diagnostic tools that I had. But, for seemingly no reason, it restarted but with only a trickle of fuel form the IP return line.

Took it for a test drive and died after driving about 5 miles. After draining the battery trying re-start and re-checking my fuel line I let it sit for a couple of hours.

When I went back and it eventually re-started, again for no apparent reason. I am now thinking I have an electrical fault at a ground somewhere.

So, where should I go from here? Thanks for everyone's help.
Peter

My pertinent information:
1.Lift pump test - Brand new lift pump because I didn't see return fuel flowing out of the IP. Tested per the link and OK.
2.Service Engine Light - Off
3.Model year - 1995
3a. Odometer reading - 181500
3b. Miles on major engine components if been replaced - IP Replaced at 40k per PO.
4.Do you have any engine codes? No.
5.If you do have engine codes - what are they? None.
6.Air Filter condition (visual check) - K&N but looks relatively clean.
7.Fuel filter condition (freshly changed or condition unknown) - Changed with new lift pump.
8.Condition of Battery terminal connections (cleaned and tightened) - Yes.
8A.Known condition and age of the batteries. - Strong charge, looks to be 5 years old (72 month Interstate Batteries)
8B.Are batteries of differing age or are they a matched set? Matched
9.Upon cold start, does the radiator hose get hard quickly? No.
10.Upon cold start, do you have excessive white smoke? Always a puff of white smoke, but not prolonged.
11.Do you have excessive cranking time before the engine starts? Lately, yes.
12.Have you used the block heater, and does this effect engine starting? Block heater did not improve starting performance.
13.During hard acceleration, do you have excessive black smoke? No.
14.Do you have any unusual exhaust smoke issues? No.
15.Turbo check out - I haven't been able to keep it running long enough to check but I don't suspect any problems.
16.Do you have an EGR on the engine? (An F or an S engine code) Yes, but the vacuum line was removed by the PO.
17.Indicate if you know if it’s a 1500, 2500, 2500HD, 3500, 3500HD. Yukon 2DR 4x4, I believe that make it a 1500.
18.Indicate fuel that you are using: Bio-diesel, #2 Diesel, #1Diesel, SVO/WVO, other. Problems on Pump diesel (#1 or #2?) Works great when I can switch to WVO.
19.Are you using any fuel additives? If so, please list. None.
20.Please indicate geographic region you are in: (example: Texas or Canada) North Carolina, problems began when it turned cold (30's at night)
21.Do you have any service history available that might pertain to the problem you are having? No.
22.Please indicate any modifications to the vehicle that might help us diagnose better. Snorkle-ectomy, WVO conversion (dual tanks, lift pumps, and filters, ie. two complete fuel circuits until the IP.)
23.Upon unscrewing the fuel cap, do you have a large vacuum formed in the tank? There has been a vacuum, but I keep the cap loose to prevent this.
24.Location of PMD/FSD? On the pump/remoted over intake/remoted out of engine bay (please specify specifically) Heath relocation kit and new PMD, on diff skid plate.
25.Are ALL glow plugs in proper working order as per this thread? I have not checked this yet.
 

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Good post, excellent info - really sounds like the in-tank sock filter is plugged up.

Or the tank-switching valving is not totally functional.

Pull the lift pump fuse - up by the LP Relay, connect it to the battery, open the t-drain, see how it runs with engine off.

That applies power directly to the LP so you can check fuel supply capacity

Try your switching scheme while testing.
 

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Discussion Starter #3
LP Power

Pull the lift pump fuse - up by the LP Relay, connect it to the battery, open the t-drain, see how it runs with engine off.
Does the lift pump care about the polarity? I'll try this tonight!

I'll check the tank condition.

Will keep you posted.

Peter
 

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One terminal is the PCM driver - don't care when the Ign is off, won't spark when +12v power is applied.

Other terminal is the lift pump, does spark when power is applied.

I pull the fuse, attach power to one fuse spade, then stick the other spade into the lift pump terminal in the fuse-holder.
 

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Discussion Starter #5
Which is the lift pump fuse?

I checked the fuel pickup and it is very clean, even with cold temps lately, (20's at night, 40's day) no waxy build up.

I took it out for a spin as it started immediately (much better than the past) but not 2 miles away it completely died. No DTCs.

I went out to jump the fuel pump like described above but I could not find the lift pump fuse.

After a long crank time it eventually restarted and I made it back home.

I checked my manual for the fuse. From the schematic, the ECM-B 20-amp fuse is the one I'm looking for.

Do I have that right?
 

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The lift pump fuse is on the firewall, next to or near the lift pump relay - a rubber rectangle with two orange or red wires in the bottom
 

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Discussion Starter #7
The lift pump fuse is on the firewall, next to or near the lift pump relay - a rubber rectangle with two orange or red wires in the bottom
Wow, it might be a snake about to bite me, but for the life of me I can't find it.

I attached a picture of my fuse box, the lower left hand corner says "Fuel Pump Relay". Different relay?

-Peter
 

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Yeah, I think they did start heading towards OBD2 in '95, with the underhood fuse-box\power-center - that must be it, with the ECMB fuse as supply.

The FSOL fuse is for the IP - ESO and FSD\FS
 

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Discussion Starter #9
95 Hybrid

OK, good, I'm not crazy.

GMC must have done a half and half becuase this truck is OBD-I yet has the underhood fuse box.

The truck is dead and will not start a block away from the house. I'll go back and try the "jump the lift-pump" trick you recommended. If it doesn't operate the pump, what's next?

-Peter
 

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My 95 has the same fuse box.

You either jump across the pin socket under the lift pump relay or you can pull the OPS plug easily enough and short across the connections for the orange wire and gray wire. The orange wire should always be hot. The gray wire goes directly to the lift pump.

Also you might try putting the truck in drive with the engine not running and turn the key to start and see if the pump runs.

Lift pump relay needs to be in for both start and run power to lift pump.
 

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Discussion Starter #11
The "jump the lift-pump" did not make the fuel pump operate. Yet, when cranking fuel flows from the t-valve.

I went "probing" at the engine harness per the "Cranks but does not start" page in the service manual.

Turns out there is no voltage (1.2V) at the 'A' terminal while cranking. According to the manual it is either an open or grounded circuit, a bad connection to the PCM, or the PCM itself.

Any ideas how to narrow it down?

Thanks again...
Peter
 

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First jump from the orange wire to gray wire on the OPS connector. If it runs then you can check for fuel when it's running for proper operation.
If you get it started and pump does not run then the OPS is bad.

As far as starting goes you have good fuel for that.

I'm lost on the other stuff for now but you sound like you've got the fuel thing squared away.
 

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OBD1 LP runs only in START and when oil pressure closes the OPS - you'll have to manually energize it with a jumper from +12v.
 

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Discussion Starter #14
I am definitely getting fuel to the IP when cranking...verified by pulling the fuel line off of the IP inlet while someone cranked.

After checking electrical connection over and over again fuel started to trickle from the oultet of the IP and bingo, it caught and started.

I was able to drive it home.

While it was still running I pulled off the return line and at idle only a trickle of fuel, probably 1/4 the volume of the supply fuel was coming out of the IP. Could this be a fuel pressure problem?

I plan on checking the fuel pressure next.

Any other ideas?

Thanks,
Peter
 

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With it running open the drain valve, fuel should come out.

If it does not and the lift pump can be heard running the filter or line is restricting fuel flow or the lift pump is bad.

If the pump is not running and you can make it run by shorting the orange wire to the gray wire on the OPS connector the OPS is bad.

If it does not run when the wires are shorted together the relay may be bad, no 12 volts to orange wire, or a problem in the gray wire from OPS to pump.
 

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Discussion Starter #16
With it running open the drain valve, fuel should come out.
It does, at a 1/2 pint in 15 secs.

But only a trickle comes out of the injector/IP return line while running. I would think much more would flow out.

And when I have a no-start situation, no fuel comes out of the injector/IP return line. BUT, the LP is still pumping a strong flow of fuel.

I gues that's why I'm leaning towrads pressure BUT I have a brand new LP and OPS and filter.

Thanks...
 

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If you have a minimum of 2 psi under load that's good. At extreme throttle up it may drop near zero for a second but normal running you would be OK with 2 psi or greater.

I normally rev up the engine with the filter drain valve open. If fuel flows the fuel supply is good.
 

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I guess the thing to check now is if the fuel supply is gone when it dies. It sounds to me from your fuel flowing while trying to crank that it's something other than fuel supply.

As stated earlier though when it dies while driving maybe it's a problem with the valve or something with the OPS. Leaving the starting problem to be something else.
 

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j.peter.b;1573098; said:
I am definitely getting fuel to the IP when cranking...verified by pulling the fuel line off of the IP inlet while someone cranked.
j.peter.b;1573482; said:
It does, at a 1/2 pint in 15 secs.




This indicates no fuel problem that would keep it from running. The IP being electrically driven it seems that it's something along the line of your normal electrical problem stalling and stumbling.
 

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Discussion Starter #20
I guess the thing to check now is if the fuel supply is gone when it dies. It sounds to me from your fuel flowing while trying to crank that it's something other than fuel supply.
Yes, I agree. When it's dead and not wanting to start, I still have fuel flowing to the IP

As stated earlier though when it dies while driving maybe it's a problem with the valve or something with the OPS. Leaving the starting problem to be something else.
If by valve you mean the switching valve for WVO, that might be something. I plan on re-connecting the factory lines and not having any valves in the circuit.

If you have a minimum of 2 psi under load that's good. At extreme throttle up it may drop near zero for a second but normal running you would be OK with 2 psi or greater.
That's good to know. I plan on a road test to observe the fuel pressure.

I normally rev up the engine with the filter drain valve open. If fuel flows the fuel supply is good.
I'l try that too.

Thanks,
Peter
 
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