Possible fuel issue

[Keith_J]

Looks clean. Metering valve looks like it moves freely. Try bumping idle speed, external screw on throttle arm. Realized with pump installed, you cannot spin it with a drill to see governor function. Thermal compensation looks ok.

 

[Erik Unyielding]

Looks clean. Metering valve looks like it moves freely. Try bumping idle speed, external screw on throttle arm. Realized with pump installed, you cannot spin it with a drill to see governor function. Thermal compensation looks ok.

So don't change out the thermal spring? Idle speed seems good when it's cold, hard to tell without a tach but it doesn't seem low or high.

 

[Keith_J]

If the thermal compensation leaf is solidly attached, there is no reason to replace without further testing. If you had a soldering iron, using it to heat the leaf would be a positive test.

Beginning to think the prior owner adjusted the external idle speed, not knowing about the cold idle speed solenoid. But this is a replacement pump as 1986 originally had 4520 pumps. Federal Supply System used 4521 replacement pumps for use of JP8.
1984-85 used the 4267 calibration which had the flex ring problem. 1984 also had 4305..

No need to install a tachometer if you have a digital multimeter that measures frequency as stock alternators have a tach terminal, driver side alternator is usually connected to a brown wire, just a push on boot.
The crank pulley is 7.75 inches in diameter, alternator is 3 inch so the alternator spins 2.583 times faster, at 625 RPM crank, the alternator spins 1615 RPM, 27 revolution per second. 27SI is a 16 pole rotor so 8 cycles per revolution or 215 Hz corresponds to 625 RPM crank.

 

[Erik Unyielding]

If the thermal compensation leaf is solidly attached, there is no reason to replace without further testing. If you had a soldering iron, using it to heat the leaf would be a positive test.

Beginning to think the prior owner adjusted the external idle speed, not knowing about the cold idle speed solenoid. But this is a replacement pump as 1986 originally had 4520 pumps. Federal Supply System used 4521 replacement pumps for use of JP8.
1984-85 used the 4267 calibration which had the flex ring problem. 1984 also had 4305..

No need to install a tachometer if you have a digital multimeter that measures frequency as stock alternators have a tach terminal, driver side alternator is usually connected to a brown wire, just a push on boot.
The crank pulley is 7.75 inches in diameter, alternator is 3 inch so the alternator spins 2.583 times faster, at 625 RPM crank, the alternator spins 1615 RPM, 27 revolution per second. 27SI is a 16 pole rotor so 8 cycles per revolution or 215 Hz corresponds to 625 RPM crank.

I'll see if I can dig around for my multi tester. Headed out on a quick getaway this weekend so I'll likely report back in a few days.

 

[Squibbly]

Replace the small length of black rubber fuel hose in front of the air filter going into the IP with clear fuel hose.
If the truck starts stalling go look at it and see if you've got air bubbles or no fuel coming in.

Just a thought.

 

[Erik Unyielding]

Replace the small length of black rubber fuel hose in front of the air filter going into the IP with clear fuel hose.
If the truck starts stalling go look at it and see if you've got air bubbles or no fuel coming in.

Just a thought.

I think that's worth exploring. It would be weird if the IP was only pulling air when hot though, right? I've run into air issues on small DI diesel engines that don't have a lift pump and rely on the IP to pull fuel all the way from the tank but when there's a leak, they run like crap all the time, not just hot.

Also, what are the odds that it's an inop fuel cap vent that's creating a vacuum in the tank after running for a while? I've had that on the smaller engines I mentioned when their tank vents get stuck. I suppose the easy way to test is to drive it for an hour with the cap off but I'm just wondering if anyone has ever heard of that happening on these trucks.

 

[Squibbly]

I think that's worth exploring. It would be weird if the IP was only pulling air when hot though, right? I've run into air issues on small DI diesel engines that don't have a lift pump and rely on the IP to pull fuel all the way from the tank but when there's a leak, they run like crap all the time, not just hot.

Also, what are the odds that it's an inop fuel cap vent that's creating a vacuum in the tank after running for a while? I've had that on the smaller engines I mentioned when their tank vents get stuck. I suppose the easy way to test is to drive it for an hour with the cap off but I'm just wondering if anyone has ever heard of that happening on these trucks.

I'm not sure that diesels can get vapor lock, but yes perhaps as the tank empties it isn't venting from the cap as it should, and creating a vacuum situation, which is going beyond what the lift pump is capable of overcoming. Seems reasonable, and would "seem like" a heat issue when it's simply the gas tank draining over time.

Point is, that spot in the fuel system is a good place to determine if the issue is pre or post IP. Easier to troubleshoot fuel issues IMHO when you are able to actually see what the fuel is doing at each stage.

 

[Erik Unyielding]

You guys, I think I found and fixed the issue and I feel really dumb about it. While crawling around under my truck today and looking for places to eventually mount my spin-on filter heads and electric pump I spotted a previously unnoticed inline fuel filter with a suspiciously collapsed-looking filter element visible through the clear housing. I went out and grabbed another at the store, swapped the old one out, and went for a 5 minute ride to a nearby parking lot. I idled in the parking lot for 50 minutes which was previously more than enough time to get it to misbehave and it ran beautifully. I really put my foot in it on the ride home, just to be sure, and I had no power loss, surging, or stalls whatsoever. I guess I should have fallen back on my reefer tech days when a fuel issue diagnosis started with simply inspecting everything from the tank to the IP.
While an embarrassing failure on my part to recognize a simple fix, I'm really glad I posted here about it. I learned quite a bit from this thread, thank you all for your responses.

 

[cucvmule]

If there is a fuel filter under the truck I would suspect that the fuel tank has allot of crud in it which is why the filter is there before clogging the main firewall filter.

After reading your symptoms the fuel filter is my first go to for the symptoms you described. As the truck is in motion the filter gets saturated with crud. After the truck is off some of the crud will fall from filter and will let some fuel pass until it clogs up fully again. As truck loses power blue smoke begins to get more dense.

The clear hose on the return line is a great indicator of air entering the system but does not tell you where from, and that is reason enough to begin to replace all rubber fuel line. As Barrman suggested replacing simple filters and fluids by you will give you a baseline to begin from.

Keep a journal of all maintenance and repairs that you do from now on, day year and mileage of repair. I do this for my own equipment so not to rely on memory for answering questions that I can refer to a ledger for answers.

I would drop the fuel tank and clean it out to make sure you do not have to continue with filter problems, or keep an extra filter for under truck and will be a miserable experience changing on the road.

I have 2 spin on fuel filters for my diesel trucks, 1 5 micron and 1 1 micron and if the tank is contaminated usually the 5 takes the brunt but eventually the one gets dirty also. But for time and money dropping the tank is the best maintenance you can do. The least amount of rubber in the fuel system the better. Replace all rubber fuel lines and clamps, and get the short hose at the cab to frame junction behind the fender well tub. The hose is the least accessible and always forgot about, and always creates havoc at the worst time.

Good Luck and Detroit's are the best.

 

[Keith_J]

Ouch..I assume when these vehicles are straight from the military..yet again this assumption trapped me.

Glad you got it fixed easily

 

[cucvmule]

Keith_J you provide a great wealth of information that is used and thought about on the forum. Thanks

 

[Erik Unyielding]

Ouch..I assume when these vehicles are straight from the military..yet again this assumption trapped me.

Glad you got it fixed easily

I assumed the same and didn't look for an inline filter because it isn't in the TM. I should've known better because when I was training new techs at my last job, one of the rules I gave them was "never assume a system is unmodified and never trust the last guy's work".

 

[Squibbly]

I assumed the same and didn't look for an inline filter because it isn't in the TM. I should've known better because when I was training new techs at my last job, one of the rules I gave them was "never assume a system is unmodified and never trust the last guy's work".

Nice. Glad you got it figured out.