Non-Mercedes 6.5 GM diesel problem - PeachParts Mercedes-Benz Forum

kerry · #1 10-07-2013, 03:54 PM

Got a thread on this over at the 6.5 forum at Diesel Place but I thought I'd pick people's brains here too. About 3 weeks ago I bought a 93 GMC Sierra pickup with a 6.5 turbodiesel and 104k miles. Started it cold and hot before purchase. Fired right up. Finished up the third of tank of fuel that was in at purchase, filled it up and ran 400 miles on that tank. Filled up again. Immediately it began showing symptoms of harder starting and 130 miles later it is extremely hard to start when hot. It does start if I pour water on the IP to cool it down, which is the standard test for a worn out pump on these IP's. But I'm suspicious of that diagnosis since the hard starting occurred immediately after filling up. I've replaced the fuel filter. Glow plugs are controlled by a temp sensor in the engine and don't operate once the engine is up to temperature. I installed a fuel pressure gauge and lift pump is providing about 7-8psi all the time. Bleeder valve shows plenty of fuel when cranking or running. I was wondering if I got a tank of thinner winter fuel and it showed up the problem of a worn pump right away over summer fuel. Other than the hard starting it runs fine.

jusme · #2 10-07-2013, 04:50 PM

Do you have the remote mounted heatsinked PMD on it?

kerry · #3 10-07-2013, 05:10 PM

No, it's a mechanical pump. DB2.

kerry · #4 10-07-2013, 05:33 PM

Glow light is not working during the hard starts, but the whole time I've owned the truck, the glow light has stopped coming on once the engine is warmed up, even prior to the hard starts. I'd been told that was normal on a GM 6.5.

Skippy · #5 10-07-2013, 05:54 PM

From what I dimly recall of mechanic school a dozen years ago, no glowing when hot is a "feature" of the 6.5. Sounds like you have a bad pump. Last time I checked Stanadyne rotary pumps were relatively inexpensive. That one should have lasted longer, but maybe it was run on petrol or other bad fuel before you got it.

1project2many · #6 10-07-2013, 06:16 PM

The glow plugs usually cycle some while the engine is first running. The temp sensor can be faulty and cause short glow cycles. Combined with a few bad glow plugs this can cause a hard start. But a good 6.5 in mild weather usually starts with minimum amount of heat anyway. Glow plug problems show up when the engine gets cold.

Cranking speed OK? Diesels hate cranking slowly. Hot starters can slow down. I'm sure you'd recognize that.

Air leaks occurring due to temp change when engine's warm? Clear return hose on IP can help identify air intrusion issue.

Problem occurred after fill up, leads me to wonder if this is an existing problem the PO discovered a way around with a fuel additive. I used to work on a 6.2 powered truck that had fuel which always smelled a bit like gasoline. Owner was adding gas to help a hard start. Yikes!

It's possible to connect an injector to the injection pump to observe pattern and flow. Also, watch the return line from one bank of injectors. If no fuel during crank or mostly bubbles, and air is not the culprit, expect to change pump.

kerry · #7 10-07-2013, 06:23 PM

Glow light does cycle a few times after starting when cold so I think it's working correctly. I haven't ohmed out the glowplugs but it seems to start ok when cold. Starter speed seems fine. It's definitely not turning any slower when hot than when cold.

stukreit · #8 10-07-2013, 07:22 PM

100% operational glow plugs are important, and the older glow plugs have
a nasty habit of swelling and getting brittle, breaking off while removing.
So check that all 8 have the spec resistance, the connecting wires
are clean, the oil pressure switch (if your older motor has this) and the
glow plug relay are good. Also, do a proper load test on the batteries.

Re. fuel delivery, air in the lines can be devilish. Try cracking some injector
connections loose to get some extra bleeding. and check that fuel return
lines aren't kinked or crummy. Your fuel filter is different from mine. Does
it have a built-in collector for water? drain that thing.

Good luck. It took me 2 years, several PMD's, finally 1 remote PMD, 1
lift pump, and 1 IP to get my 6.5 Suburban sorted out but now its super reliable.

kerry · #9 10-07-2013, 07:30 PM

I haven't checked the resistance on the glow plugs but I'm virtually certain everything else is workind and when the hard starting occurs the engine is warm enough that the glow plugs aren't called for. New batteries and starter is spinning fast. OPS is working and lift pump flow is confirmed. Water drain has been activated (that's the bleed valve on the front of the engine I believe). Air in lines is a possibility but why only when hot?

vstech · #10 10-07-2013, 08:22 PM

on mine, it turned out to be CRAPPY injectors.
I know the set under the turbo are a pain to get to, but pulling them, and popping them is a great idea. the GM injectors have a copper crush washer, not a heat shield like the MB's do...

kerry · #11 10-07-2013, 11:53 PM

John, you were getting hard hot starts with bad injectors? What about the big difference in starting between the two tanks of fuel? I can't think of any way to explain that besides a worn pump.

sixto · #12 10-08-2013, 01:20 AM

Our '97 Suburban 6.5 had a glow plug wire disconnected for a while. It didn't start differently with and without that glow plug but it doesn't get that cold here. No CEL or codes through the wait to start light.

Sixto
87 300D

1project2many · #13 10-08-2013, 07:21 AM

You can spend the time to resistance check the injectors but it's much easier with test light to B+. I worked in truck country for 7 years, 5 in a GM dealer, and I've only seen a couple of glow plugs with wrong resistance that weren't completely open. And those showed up as a very dim test light.

I can head out now and start our 200k mile vans in 60deg weather without waiting for a full cycle of the GP's. I've seen newbie drivers hop into a van with a warm engine and turn the key right to start... and the van starts right up.

If you think the issue is due to a worn pump and fuel viscosity, you could try connecting a can containing four gals fuel mixed with a gallon of MMO to the fuel line. Start it cold, run it till warm, shut it down.

Here's a question: Is time a factor? IOW, if you shut the engine down hot then try a restart withing 10-15 seconds, will it start? Is the problem worse the longer you wait?

Did you check the lift pump output while hot? If the pump works cold but fails to work warm the IP may not be able to draw fuel through it. The IP can lift fuel a foot or so in most cases but don't count on it to happen.

Another no dollar test is to get the rear of the truck as high as possible. Air bubbles like to be at the top of a system and I would sometimes leave a hard start on the twin post lift with the rear axle raised to see if a hard start failed to occur.

Some guys like to add a check valve near the tank to prevent fuel drainback. It's not the right way to fix but it's good enough for some.

Any way to talk to PO about what he ran for fuel? I'm still betting he found an additive that helped and went with that instead of paying for a repair.

JB3 · #14 10-08-2013, 08:56 AM

im putting my money on a worn injection pump after you test the glow plugs.

Sounds exactly like a school bus powered by a 6.5 I worked on a few years ago. Tough starts always, but once running, you could never say there was something wrong, just ran great after you finally managed to start it.

We tested all the glow plugs, replaced them all for good measure, tested all the injectors, and finally pulled and swapped the pump, which is really easy by the way on a 6.5.

That solved the problem, chances are your fuel lubricated injection pump has suffered a little from ULSD. When we pulled the old injection pump apart, it was full of galled surfaces and other worn areas.

kerry · #15 10-08-2013, 08:57 AM

I think it will start within a few seconds but much longer and it just cranks. Lift pump seems fine under all circumstances, at least it always shows pressure on the gauge. If it was an air leak problem, it seems it wouldn't start once it cools down either.
I should add that according to what I've read on numerous sites, problems with a hot start are typical of a worn IP. Here for instance:
Kennedy Diesel
The previous owner, by all appearances, was a stand up guy. I doubt he had tried any additives. He told me things that were wrong with the truck that would not have been evident otherwise.
One thing I don't like about this GMC is that there's no simple way to test the system with an alternate fuel supply. Metal lines in and out of the lift pump and the rubber line to the fuel filer very hard to access between the bell housing and the firewall.

Not sure what you mean by 'test light to B+'.