IP behaving erratically after a LOT of troubleshooting

[gavio]

Hi All:

OK, after a month of trying to troubleshoot a problem I've got on this truck I'd borrowed, presumably so I'd have something to drive while I fix my van........I'm about to give up, but maybe some of you have some fresh ideas.

The truck is an '83 K10 with a stock 6.2.

It behaves very erratically with a lot of surging and bouts of rough idling and timing advancing itself irregularly. A clear piece of reurn line shows that a lot of air is coming through the IP but here's the deal:

Even cutting out virtually ALL of the supply system, cutting down to just a piece of fresh fuel hose directly feeding the IP from a clean gallon jug of fuel, I'm still getting air coming through the IP, but the IP does not leak a drop of fuel - the valley is dry as a bone.

Where is this coming from? Is this some kind of pump failure?

The truck is still driveable just barely and I've already done all the expected stuff - changed filters, repaired hose connections, etc. I'd like to get on with fixing my van, so I need to decide quickly if it's time to just give this thing up as "beyond me."

Thanks for any help you cats can offer.

[NH2112]

Sounds a lot like my truck - I have an electric fuel pump hooked directly to the tank by a length of rubber fuel line, and I took my secondary fuel filter out of the system due to a leak. I have to run mine for several minutes when I first start it, and it runs rough and almost dies (sometimes it does) during this time. My thought is that the electric fuel pump is letting air leak in when it's off (which whould explain the rough starts), but the timing changes, surging, etc, are a little harder to solve. I'm thinking air's leaking in through 1 or more injector nozzles.

Right now I'm really tempted to drop my project truck engine into the C1500! :D

[ 07-15-2005, 04:25 PM: Message edited by: NH2112 ]

[gavio]

Hmmmmmmm

Air through an injector nozzle - or line? I haven't heard of that. It could explain why there was air coming through even when I had nothing in the supply line but a couple feet of hose and a jug of fuel (thus ruling out lift pump, filters and tanks).

Anybody else heard of this? Know what to do to track it down?

[john8662]

It's possible to have an injector thats letting air in the system, although this is somewhat rare. Unfortunatly, the way to track it down is by removing them and having them all tested for leak down.

Double check all the return lines also, a return line, even one on an injector will cause air in the system.

I suspect a fuel delivery problem to the injection pump, possibly a cracked line or a mushroomed connection letting air in the system.

It's good that you have the clear line on the injection pump, that really helps to troubleshoot the problem. Every time I've had a hard start situation on one of these engines it turned out to be either on the return side or on the supply side of the injection pump, and always a rubber line.

The fuel pump could sure be suspect as it has a diaphram that could rupture, but usually they go to leaking outside or leaking fuel into the engine oil.

Being that you have a van, I'd verify the connections on the back of the secondary square fuel filter, also making sure that the fuel filter is making a good seal on the fuel filter housing (that the rubber on the fuel filter isn't split or contaminated).

Don't give up on this!

[gavio]

Thanks for the input.....

to clarify, the problem isn't with my van (it has a bad head gasket)........the problem is with the truck I borrowed to drive while I fixed the van.

So far, I have just spent a bunch of time trying to fix the truck - not good!

Right now, I am running the truck with an electric lift pump drawing fuel through the primary filter and feeding directly to the IP. It seems to start easily, with very little cranking, and usually idles fine - as RPMs go up a bit, that's when things get chaotic.

Anyway, any more thoughts are welcome.

[twaddle]

Hi Gavio,
You seem to have checked out everything right down to running on a separate fuel container to rule out any problems in the fuel tank.

Air should not get into the system between the filters and the injection pump as this side of things should be working at a positive pressure not a vacuum which draws in air.

You say "as RPMs go up a bit, that's when things get chaotic".

Are you getting air through the clear return hose as you increase the revs?
If you are getting air through the return hose? As you have checked just about everything else I would say that you have at least one faulty injector which is not holding cylinder pressure and allowing blow by/cylinder pressure into the fuel system.
In almost 30 years (time flies when you're enjoying yourself) working on diesel engines I have only come across this a couple of times, but it can happen. Sometimes if you get the revs to the point when it gets "chaotic" you can pick up which injector(s) is/are faulty by loosening each injector pipe at the injector. Where the engine note changes even more the injector is usually ok and when it does not change is usually where the fault is. Sometimes you will notice the faulty injector appears to have more air in the fuel when the pipe is loosened.

The next thing is get the injectors tested although the test cannot test for cylinder pressure blow by and the injectors can be appear to be operating satisfactorily when bench tested.

Good luck and best regards

Jim Twaddle
Biggar, Scotland

[BobND]

I have seen the "sock" on the fuel tank pickup get restricted.

Is it a 2-tank system with an electric valve? (These can fail, and only partially switch and cause weird problems.)

Electric fuel pumps PUSH fuel better than the PULL (or suck) fuel, and should be located as near the tank as is practical, with only a pre-filter ahead of them, and the main filter on the OUTPUT side of the pump. With the pump back in the frame rail, you will eliminate a lot of possible vacuum leaks, and any leaks that remain between the electric pump and the IP will now be under pressure, and should become obvious by leaking fuel.

You need to "tee" into the line just ahead of the IP, and check the fuel pressure under loaded, driving conditions. I believe you will want to see at least 7 to 9 PSI just ahead of the IP at all times.

[gavio]

Thanks all, for the interest.

Jim: I will see if I can try to isolate an injector problem such as you describe - it certainly is something I've not thought of before.

Yes, what I meant was that when I raise the RPMs to maybe 800 or so, the engine seems to grab a bunch of air from somewhere, the timing advances (I presume due to the lowered IP housing pressure), the RPMs then surge ahead jumping to like 1700 or 1800 (I'm estimating since there's no tach) and then I see new air bubbles in the return.

Does this sound like what you are describing?

Thanks again.....

[More Power]

Drawing (sucking) fuel through a Stanadyne model 80 fuel filter can be problematic, in that there are numerous places for air entry. This problem can be made much worse if there's any sort of restriction between the fuel in the tank and the filter (clogged sock, faulty tank switcher, kinked hose, etc.).

Pressurizing the fuel filter is the standard nowadays, though this doesn't lend itself too well to the model 80.

Jim

[twaddle]

More Power,
In Gavio's 1st email he says he has run the engine taking the fuel from a jug with a hose directly onto the IP, but it still drew air and ran badly.
This should have eliminated all other sources of possible "drawing of air" into the system.
All other possible sources of air ingress would have been eliminated if the hose was directly onto the IP.

The only thing I can think of is that if the jug was situated too low down this could cause cavitation within the IP. If I test an engine using a seperate container directly onto the IP I usually locate the container high up to give a positive gravity feed and eliminate the danger of drawing air.

Gavio, Where and how high up was the jug when you tested the engine and was the hose from the jug connected directly onto the IP (with everything else disconnected)?

Regards

Jim
Biggar, Scotland.

[gavio]

Hi Jim:

The jug was propped up on the inner fender, kind of squeezed between the A/C box and the hood hinge - in other words, at about the level of the IP. Then I just had like 2 or 3 feet of hose, connected to the little piece of hose that lives permanently under the intake manifold. I had considered that there could be a leak in that permanent hose, but with the fuel flowing openly from the container over such a short distance, i figured any leak capable of forcing the IP to suck air under so little restriction would just HAVE to be a major leak that would drip (or pour) fuel out all the time - such is not the case as the valley of the engine is dry as a bone.

Now, here's where it gets weird:

After I put it back together with the abbreviated fuel system I am currently (temporarily) running, it ran badly for a day, then it had a brief journey into not starting, which showed the symptoms usually associated with a fried fusible link at the starter - I messed around with it, just kind of fiddling as the link wasn't really fried and lo and behold.....

The truck started and ran perfectly for a day - admittedly a short day as I only drove a few miles.

Next day, ever puzzling, it ran badly again......

And today, it's running pretty well - so far at least.

Could it be that the IP was having trouble purging old air for some reason? My van purges air pretty quickly and will recover to running perfectly in just afew minutes even from a complete airlock, so that is what I am used to. Could other factors make this pump behave differently - could something make it recover THIS slowly? And then, of course, from where would it have gotten the air in the first place, since that came on suddenly BEFORE I changed the filters. The only notable event which had preceeded this was that the truck did run out of fuel (fuel gauge doesn't work and my friend had not notified of this fact when I got the truck from him), but I had refilled it and driven it for a week or so without incident before the trouble started.

Sorry for the long post, but this is all kind of different, if you know what I mean.

Thanks so much for applying your considerable knowledge to this.

[twaddle]

Hi Gavio,
When you get signs of air showing in the clear return pipe is it large air bubbles or lots of tiny bubbles or very few?

The short hose at the IP; if there is any doubt then you must test it or change it.

I think the test with the jug may have been set too low.
To do this test in order to eliminate all of the suction side of the system the IP must have a good positive feed with the jug or can sitting higher that the IP.

I presume that the engine behaviour changes while increasing the revs with no load as well as under load?

I had a fault similar to this some time back with my 85 M1008 pick up and it turned out to be the fuel suction hose that had rubbed against a cross member and cut a slot in the hose which would sometimes open and draw air into the system causing the revs and power to change and most of the time the hose seemed to close and ran without any problems.
You could have a similar fault?
Or
The other thing that is possible is that when you have run out of fuel the suction pipe in the tank has picked up dirt from in the tank.
The fact that you have what appears to be an intermittant fault may be a piece of dirt floating or moving with the fuel and restricting flow at the suction pipe.

If the jug to IP test is done properly this would let you see if the problem is in the suction pipes, hoses or tank.

Out of curiosity is the truck running on diesel or veggie oil?
The other thing I was wondering was; what is the owner of the truck saying to all this trouble you are having with his truck, you did say it was on loan?

Regards and good luck.

Jim Twaddle
Biggar, Scotland

Just one last thing, has there been any signs of water in any of the fuel?

[gavio]

Hi Jim:

Lotta thoughts, here goes:

My friend has been out of town, so he does not know - he had not been using the truck much, just driving it once in a while to keep it charged up, etc. He doesn't know much about vehicles and my original intention was to just go ahead and do the filters and whatnot, which I figured would fix the problem and it would be a favor to him, since he did loan me the truck and all.....but now I'm invested :D

The problem was happening on veggie and diesel both, so I pretty much eliminated a blockage in the tank or primary filters or anything in between, since the odds of both clogging the same at the same time seemed pretty thin.....

So we're down to the part of the fuel system that they both share - I'm currently running my abbreviated fuel system as described above, so no veggie at for now.

When I did the filters, I did find nasty rust collored stuff in the bottom of the primary diesel filter, assumed it to have been sucked up from the tank in the running-out-of-fuel incident. The secondary filter seemed normal with just a bit of gook in it when drained out completely. I tried using the water drain valve to siphon gook out of the bottom of the tank, but only got clean diesel fuel out (a couple gallons worth) so if there's gook in the tank, it seems gone now. Perhaps what was there all got sucked up into the filter - If some gook or (gasp!) water got by the filters, why would the truck then run fine for a week or more before the problems showed up?

The piece of dirt theory seems to make sense - is it possible that it has cleared somehow.......as the truck is running about right again today......

I did not realize the jug had to be higher than the IP in my test........hmmmmmm

Anyway, thanks again for all the insight - I've learned a lot this time around, for sure......

[jonflies]

Gavio,

Having very similar problems with my 82 suburban. What did you finally find?

[gavio]

Actually, Jon, I finally solved the problem by returning the truck to its rightful owner :rolleyes:

I never did get it to run consistently, but finally suspected that it needs an IP rebuild. I had borrowed it to use while I fixed my van and was getting too distracted by it so that it was keeping me from getting my van done. I recommended that the owner sell it as is (he's planning on selling it anyway) and tell prospective purchasers that it needs a fuel system overhaul.

Sorry, i know that wasn't very helpful. I would add that I had a very similar problem a while back with my van, but that turned out to just need a new fuel filter and the problem went away.

Good luck!