Idling and running rough
 

83 K20 Turbo 400 6.2L

A little background before she started acting up yesterday.

This truck was used on the oil fields until about 4 or 5 years ago. A friend of mine had purchased it for farm work. It's is in very good condition physically considering all the abuse it's most likely seen (especially since my friend and I live in Montana. So I decided to take this truck off his hands. It had a hard start. 6 glow plugs were bad and the relay was worn out. Replaced them and she was firing up much quicker. Checked the coolant temp sensor and it was reading around 3k ohms at 160ish degrees. Replaced it and the new one reads about 53-55 ohms at the same temperature. She had been running great.

When I was leaving his parents house yesterday, after helping them out, the truck made it just over a mile and it died at about 45 mph (she was at operating temp). Pulled over and restarted. Made it about 3/4 of a mile and she died again. Eventually she kept getting worse. A 7 mile trip took an hour.

During the trip, I noticed the oil level was a little low, topped off and no improvement. Well, on the final stretch home (off the highway and onto a dirt road thankfully), each little trip eventually restricted me to a 15-35' distance. Popped the hood and for 20 minutes I let the wind cool down the bay. Was able to make a 100 yard dash before she died again. Of course, each time I sat for a bit, I was on my phone doing some research. I didn't have water to pour on my IP, but I drove a tiny bit with the hood open and it made no progress with 40 mph wind hitting it. Since she died about the 4th time on that trip, a limping dog had more power in it's hind legs. No power what so ever. And if the throttle was pressed more than 1/2 way, she would spit out unburnt fuel out the tail pipes.

One of the weird things is that everytime I cranked her over after she died, it was an instant fire up. No hard starting. So, a few hours later when the engine was about 100 degrees, I fired it up and made it a whopping 80' and she died again. Just enough to pull away from my garage and near my front door.

So, hoping for a fresh start, I fired her up this morning. She had to take a little persuasion. She fired up and she will drive little spurts (never drove past 120') without an issue. However, she will not idle by herself now. If I add just a little to the throttle, she runs smoothly. Also, she has been spitting out unburnt fuel under about 1/2 throttle or more (granted she didn't reach more than 110 degrees at the heads, so it's possible that most of it was from a cold start).

I've tried running without the air filter today and yesterday with no luck. Also, coolant is full and I've checked the dipstick and oil filler cap several time and there are no signs of coolant (so it's safe to rule out a cracked head or blown head gasket).

I'm hoping that it's the Lift pump because, well, it's cheap and sounds like the culprit. Haha.

I know the primary fuel filter (attached to the intake) hasn't been changed in probably a few years (even though since my friend has had the truck, it never got over 10k miles put on it by him. The water separator (fuel filter on the firewall) is not the stock one. But it looks clean and the water is staying at the bottom of the filter (and both fuel lines are on the top).

I also read of the vent lines in the tanks are plugged, it could cause a similar issue that I had yesterday. So I took off the tank caps to allow it to vent and no change. Also tried ruling out bad fuel (last fuel added was about 4 months ago) by switching between tanks and I also put some Diesel 911 in it to try to see if it was fuel quality. No real change that I noticed.

It's also safe to say that the compression is fine because it ran great and then suddenly acted up.

That should be everything. I would greatly appreciate all plausible suggestions for this problem and would appreciate any tips for testing out components (first diesel, couldn't you tell. Haha). I'm on a limited budget as I recently moved and am still getting situated (and trying to find a job).

Thanks

Also, it's not my daily driver so if it takes 2-3 weeks to figure it out, it's fine by me. I just need it ready for the winter as my 300ZX doesn't provide one inch of traction on 1/4" of snow.

Fuel pressure test yielded around .4 psi with about 35% throttle. So there is low fuel pressure after the water separator. I did crack some lines to bleed the air from installing the fuel pressure gauge and of course tightened them while someone was cranking to keep air out. This makes sense that I have to keep my foot a bit on the throttle to idle. However, it doesn't make sense to me that with low fuel pressure I'd be spitting out so much unburnt fuel. I'd appreciate some experienced diesel owners opinions.

Fuel pressure is enough.

And like you say lack of fuel pressure should not result in fuel out the tailpipe.

I would look for a restriction in the fuel return line from the IP to the tank.
Perhaps run a separate line from the IP return to a canister in the truck bed for testing?

Pressure is .4, shouldn't it be around 4-5 psi at 35% throttle?

I'll try that out today if I've got time.

Miss read the pressure reading.

what type gauge was used to take it?

.4 bar is nearly 6 psi.

.4 psi, not bar.

Thought of this yesterday when talking about the issue with a friend. Is it possible that because it takes so long to achieve fuel to the cylinders, that eventually when the cylinders have enough fuel and it finally starts, it spits a bunch out that had ran down the exhaust when it was being cranked over? It's plausible in my opinion. I was looking at the unburnt fuel as an issue, but what if it was just an effect instead?

Alright, now that I'm awake and functioning (haha), a restriction in the return line would boost fuel pressure (which would benefit me at the moment), so I doubt that's the issue. I've been doing some reading and people have been mentioning testing the lift pump using the battery voltage. Could you walk me through how to do that? Didn't think it was possible with a mechanical fuel pump but apparently GM used the oil pressure sensor as a fuel shut off. If the oil pressure sensor is bad, it could explain the low fuel pressure and why it runs under light load but won't idle.

A restriction in the return line shuts down the IP.
It can in no way benefit you to have a return line restriction.

The mechanical lift pump is not affected by oil pressure-that is electric LP only.

That's good information to know about the injection pump. Thank you for letting me know that. Currently installing some new lights but I'll attempt to bypass the return line if I have time tonight before it gets dark (in about 3.5 hours). I'll post the results as soon as I can. And I didn't think that a mechanical fuel pump shut down as it is ran by the engine, thanks for confirming that as well.

Correct me if I'm wrong, but the return line you're talking about goes back into the injector pump right? The hose is about 8" long right? How would I go about running a line to a container? Do I run one hose from the injector pump into a container and a hose from the metal line into the same container? For the time being, I will replace the 8" hose I'm talking about with clear silicone hose. Also, the original line was brittle/old and tore pretty easy, if it had a pinhole that was sucking air, could that explain my problem?

I also read something about looking for air bubbles in the return line with a clear hose, so I will check for that when I have another set of hands to help keep the truck running.

You might be onto something. I tried blowing through the old hose and very little air came out the other end. Lets hope this solves the issue.

Well, air bubbles came through the return line when cranking and when I finished cranking, they recessed into the return lines. Can upload a video of them recessing and link it if you would like.

edit*

more information: I let the truck sit for a few minutes after cranking and idling. Checked the clear line and there is barely any fuel in the line (about 70% air). Is this a clear indication of a leak in an fuel line?

You need to get the truck running and observe the line for air bubbles-if they are seen while running you have an air leak between the LP and the fuel level in the tank.

I'll work on getting all the lines clear of air so I can get a better reading on the return line. I figured while I was checking the return system, id replace all the return lines from injector to injector as well as on all three ends of the metal pipe. This way I can observe as much as possible.

Really only need to see the line that exits the frt of the IP.

Extra insurance I guess. The old lines were stiff and slightly brittle so I figured why not, couldn't hurt at all.

It won't do any harm at all,just expect to see bubbles exit from the injectors-that's normal.

The rear injectors have plugs on the un-used lines so if you want you could join the 2 rear injectors together with a long line instead of the plugs.

So I can ignore bubbles heading to the IP currently with some air in the lines, but if I see air leave the IP, that's my issue most likely right? Or am I getting something mixed up?

you have the jist of it.

When idling about 1000 rpm, there are almost no air bubbles coming out of the injection pump. When cranking, air does leave the injection pump. Also, when idling there is a pool of bubbles in the line from the IP to return line. They just float and stay in the line.

Sounds good.

Did you check for any restriction in the return line to the tank?

I haven't. I was planning on disconnecting the return and supply lines to a can of diesel to help tell me that. Going to try to pick up a lift pump and secondary filter for preventative measures.

Where you put the secondary filter matters-filters should be post LP,for pre LP you should only use a screen and nylon preferably.

Yeah, the secondary filter is after the lift pump (still mounted to the intake plenum). Only managed to get the filter, but it hasn't been changed in a while so hopefully it helps some. Quick question, is there any fuel pressure before the lift pump or only after? Curious. And thank you again for the help you're providing.

Only pressure after the LP.

If any pressure or vacuum is in the fuel tank you need a new AC delco fuel cap that is properly vented for diesel use.

Ran the test with the sending (from right after the primary filter) and return line as well as installed the new filter. Still running the same. Gonna try to get a lift pump tomorrow and see if that helps (NAPA has one for only $20).

Just letting you know it might be a few days before I can install the lift pump (if I manage to grab one today/tomorrow) as it's currently snowing and my little car cannot make it to NAPA (sometimes I hate slicks).

Well, the new lift pump helped fill up the lines better. But I still cannot get the truck to idle. Still having the same issue. Idles for a minute with my foot on the throttle but after that it will only fire up for a second and then it dies no matter what I do with the throttle (not touching, 1/4 way, 1/2 way, varying, etc..).

Is the LP moving fuel?
Wonder if the sock in the tank might be plugged?

it seems to be moving fuel. It filled up the primary filter quicker than before (has a clear cover).

I read somewhere a while back (before I started this thread) that there is a part that tends to fail almost right at 140k miles (this has nearly 141k). Would you happen to know what I'm talking about?

no idea what that might be?
there is no best before date on parts besides the injectors that would be around 140k KM's or 100k miles.

managed to get ahold of a friend and she said the IP has a tendency to fail around 150k miles from what she has seen and that it can be tested simply. Remove the plate off the top of the IP and if it runs, it's the IP.

Untrue,there are many factors that determine IP logevity-fuel quality,amount of time run without operational LP,adjustmets(turning up the pump) ect.

I think its safe to assume her basis was an untouched IP with low sulfur fuel. But regardless, it's a quick and simple test.

Today the truck wouldn't run even without the throttle being pressed. Recorded a few minutes of what the truck sounds like when it fires up for a second (was using the screwdriver and wrench to keep the throttle open when I cranked).


https://m.youtube.com/watch?v=Tt9PHKQt1nE&feature=youtu.be

From the amount of the video I watched it seems as though you have a massive air leak in the fuel system to me.

I would try getting some fuel line and connecting it to the lift pump on one end and into a separate fuel container containing diesel on the other and see if you can get it to run consitently.

Crack a fuel injector line loose to help bleed the air and carefully tighten the line once the engine runs.

If it solves the problem trace the source of the air leak.

Cracked the top plate off the IP and she idled. Problem was found. Will do some more research to find any other possible culprits that share the same test results.

thank you for the help you've provided for me. You've been a good source of information and helped me learn about these trucks. Thanks again

If removing the top plate helped I wonder if you have a restriction on the fuel return side?
that can also cause your symptoms?

Try running the fuel return line into another container straight from the IP?

Tried that before removing the plate. I figured that if it was a return blockage, then an open line should yield results. Tried firing it over 5 times. It acted the same. Then I cracked the plate and she was idling on first crank. I'm just glad I found the problem. Money comes and goes, but this now saves me hours of future diagnostic work.