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GM 6.5 TD engine poof problelm

B

Bezzer

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Made a couple mistakes posting my problem .... so I'm re-posting in a proper category.

This is Bezzer with my problem. It has the 6.5TD with the F Engine (does not have the EGR valve). The engine has 79,000 miles on it. This problem started about 2 yrs. ago. At every cold start (like the next day). The proper way to start this engine is to turn the key on and watch for the glow plug light to go off and then start the engine.
As the engine starts turning over - I hear a "poof" out the exhaust pipe. Then after about 8 seconds there is a "poof" from the passenger side motorhome next to your leg. That is the turbo side of the engine. I have watched the cold start up at the engine and the rod on the new wastegate did not move but something made the "poof". I shut the engine off and waited one minute and restarted the engine and no "poof". I have checked the new vacuum pump for vacuum at the hose that goes onto the wastegate and it was 15 lbs. at idle at the cold start.
I have checked the ohms on each glow plug on that side of the engine. All four showed 0.5. Also checked the voltage to each glow plug wire unplugged with key on. All had 6 volts....as manual states.
If we go for a drive about 50 miles and stop for fuel and restart the engine...there is no "poofs" anywhere. No white or black smoke from exhaust pipe. The engine performs perfect when on a road trip.
Anyone know what this "poof" is or what's causing this "poof" after 25 yrs. without a "poof"??? Bezzer

I got a reply from Will L: Love to see pics, especially the engine so we can spot details of different applications. Hopefully we can help ya keep it running good a long time ... Bezzer- .I am working at getting you some pictures Will L.

And another reply from Twisted Steel Performance:
Inspect the exhaust system closely, signs of soot etc. Does it poof if you keep cranking it or does it poof when it does fire up?
Bezzer- It has one poof during start up coming from the exhaust pipe ( exhaust pipe is 7 foot straight from engine...no muffler) and it runs fine ...until 8 seconds later it poofs on passenger side. You can read the original explanation above. There is no more poofs after the engine is hot when we're going somewhere until the next morning when we start up to continue on our trip.

I am working on some pictures to send now.
 
Here's some pictures you asked for.
 

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Seems the glow plugs should be more like .04 ohms or so.

Near no resistance.

How old are the glow plugs?

I would get a set of AC Delco 60G glow plugs. I used to buy them at AutoZone. Surprisingly, they had the best price. Back then.
 
Thanks for the come back! My Chevy Motorhome service guide states the odometer should be approximately 0.5 ohm. Glow plugs are original with only 78,000 miles on them. Also, I checked for codes and there were no codes showing. Yes, I would get the AC Delco 60G glow plugs if needed. I'm a little skeptical taking them out. I've heard it can cause a real problem if they break.
 
There should be battery voltage to the GPs, unless, the GP relay contacts inside of the relay is getting burned. Could maybe also be corroded connections thats hooked to the GP relay.
Try disconnecting the GP harness from the relay then measure the voltage at that input connector on the relay, see that it is battery voltage, then with the GP wait to start in its cycle, measure the voltage on the output terminal of the GP relay. It should be exactly, or mighty close to exactly what the battery voltage is on the input side.
If the voltage on the input side is less than battery voltage then You need to figure why it is not at battery voltage, broke wire, corroded connector, corroded fuse terminals, etc.
If the voltage at the input side is battery voltage and only six volts going to the output terminal then be My guess that the contacts within the GP relay are burned and the GP relay needs to be replaced.
Someone with more knowledge on the subject might kick in with better advice.
 
What’s a poof? Is that a puff of smoke out the exhaust? Does this thing have dual exhaust? Usually both would go through the turbo, so I struggle to visualize poofs from each side.

Is this poof a stutter in the engine? If so, then sounds more like you have injectors leaking down and air in the fuel system.
 
Welcome to the Forum. Sorry you got a mystery noise…really hate those but, yours seems fairly straight forward.

So, to sum up: a 6.5 Diesel Turbo Diesel in a 1996 Chevy Safari Motor Home.
On initial cold starting, a ‘poof’ noise is heard out the tailpipe followed a few seconds later with another poof at the front of the motor.
On restart with a warmer than dead cold engine, even one that’s only run a couple of minutes, there’s no repeat of the noise.
Glow plugs are 78K miles old.
You’ve tested the voltage to the Glow Plugs and you’re only seeing 6volts during the glow cycle at the glow plugs.

My guess is you’re hearing a couple of cylinders coming to life after the other have managed to light w/o a proper glow cycle as in not enough voltage.

Your glow plugs should be getting 12v. If they are only getting six volts then you need to check output voltage at the Glow plug Controller. If you’ve got the right voltage there, your glow plug wires are probably suspect.

From the link jrsavoie posted, there needs to be some level of resistance reading when checking the ohms on the plugs. Zero Ohms, dead glow plug. The rest of the responses offer a pretty wide range of ‘correct’ levels but, regardless, without resistance, the plugs won’t heat.

There was a response from our old friend Robyn, AKA Missy Good Wench and it brought a smile at seeing her name and reading her responses…miss that gal. Just for grins, here’s her response on Testing the Glow Plugs themselves.
”The only sure way to know what a plug is doing (other than one that shows no movement on the needle of a VOM) is to hook the little creature to 12V and see what it does.
If it glows nice, its good, if it take a long time and only sort of glows in the center area (DULL Red or barely hot) toss the sucker in the brush and replace it.
Your makeing a real tough and technical job out of a simple good or bad issue.
If it glows good screw it back in, if it dont, toss it.
Robyn”

FWIW, I’m sure I’ve got a weak glow plug as I get a Fop Fop Fop noise at cold start up that goes away after a second. But, we’re talking POOF noise…don’t want to derail your thread.
 
Video a startup. Someone at the exhaust pipe. Post on youtube and link here.

I suspect you have a long cranking time and thus air in the fuel system. I see repairs been made and this can cause the old fuel lines to leak air. Return lines leaking air can allow the system to drain back to the fuel tank overnight. I do not discount the other advice as we are not there and have little info really to go on.
 
Thanks for your reply! No trouble --- Starts right up! Then you hear one poof out exhaust pipe....engine runs smooth for the next 8 seconds...then one poof comes out on the passenger side seems like where your leg would be sitting in passenger seat. Runs great!
The only time the fuel system was messed with was 18 yrs. ago when they had a recall for the injection pump replacement. Other than that is changing the fuel filter. And that has never been a problem.
I can turn the key off - sit for 3 minutes- starts right up and no poofs. Runs perfect and drives perfect until the next fresh start....like the next morning.
 
Bezzer said:
Thanks for your reply! No trouble --- Starts right up! Then you hear one poof out exhaust pipe....engine runs smooth for the next 8 seconds...then one poof comes out on the passenger side seems like where your leg would be sitting in passenger seat. Runs great!
The only time the fuel system was messed with was 18 yrs. ago when they had a recall for the injection pump replacement. Other than that is changing the fuel filter. And that has never been a problem.
I can turn the key off - sit for 3 minutes- starts right up and no poofs. Runs perfect and drives perfect until the next fresh start....like the next morning.
Click to expand...
I vote with WarWagon: air in the system and likely cause is leaking injectors. Once you get the air out, it will restart no problem.

You need to start with the proverbial clear return line at the IP. Also check all the return lines at each injector. At 79K miles, you are close to the recommended 100K change interval for injectors, but you also have age working against you.
 
Hi Fellows! I appreciate all the suggestions you have made. I would like to stop calling it "poof" and call it backfire - like a gas engine. After I turn the key to the on position and wait for the glow plug light to go out... .. I immediately start the engine. It only takes 5 seconds to start. Now after 4 of the 5 seconds I hear a backfire out of the exhaust pipe. The engine has only one 3" ID straight pipe. After the engine starts it runs smooth. About 8 seconds later on the passenger side I hear another backfire. If I turn the engine off and wait three minutes it starts right up after the glow plug goes out and there are no backfires anywhere. When underway the motorhome runs just like I picked it up new.

I have attached a copy of my GM motorhome service guide that stated about "Glow Plugs". At 40 degrees outside this is the exact way my engine starts.There is a factory installed electric fuel pump on the passenger side frame rail that starts running when you turn the key on. It is 12V and runs great.
After you turn the engine off....you can also hear the sound of another "possible" fuel pump on the engine that runs for about 10 seconds? What do you think that is running after turning the key off? I do not think I have a fuel problem.
Any other ideas will be appreciated!
 

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So earlier you stated you only had 6v to the glow plugs. Your manual clearly states you should be getting 12v to them. FWIW using those 6v glow plugs on 12v makes them heat faster and not last as long unless they are the self regulating kind. You need to find out what glow plugs you have and if they are self regulating.
 
Bezzer said:
Hi Fellows! I appreciate all the suggestions you have made. I would like to stop calling it "poof" and call it backfire - like a gas engine. After I turn the key to the on position and wait for the glow plug light to go out... .. I immediately start the engine. It only takes 5 seconds to start. Now after 4 of the 5 seconds I hear a backfire out of the exhaust pipe. The engine has only one 3" ID straight pipe. After the engine starts it runs smooth. About 8 seconds later on the passenger side I hear another backfire. If I turn the engine off and wait three minutes it starts right up after the glow plug goes out and there are no backfires anywhere. When underway the motorhome runs just like I picked it up new.

I have attached a copy of my GM motorhome service guide that stated about "Glow Plugs". At 40 degrees outside this is the exact way my engine starts.There is a factory installed electric fuel pump on the passenger side frame rail that starts running when you turn the key on. It is 12V and runs great.
After you turn the engine off....you can also hear the sound of another "possible" fuel pump on the engine that runs for about 10 seconds? What do you think that is running after turning the key off? I do not think I have a fuel problem.
Any other ideas will be appreciated!
Click to expand...
It’s still your lift pump running after engine is turned off. Lift pump is controlled by oil pressure switch, so it will run until the pressure dissipates.

Air in the fuel system, or glows, or both. If you don’t want to troubleshoot and are one to just throw parts at it, start with replacing glows (AC Delco) as they are the cheapest. Then injectors, but replace all the return lines, oh and do the clear return line. Do something, cross it off the list and then report back on your poof contractions.
 
I spent a month in Montana back in December. When I came home and went to start my ‘99, it started but was rough for about 30 seconds. Once that cleared, it hasn’t happened since. Conclusion: couple of the injectors leaked down. Injectors were replaced 2 years ago, but nothing is guaranteed..
 
Before you do start buying parts, I’m going to double down on what AK Diesel Driver (mainly because I’ve said it too) and really REALLY suggest you address the 6v power to the glow plugs.
Confirm voltage at the Glow Plugs is or is not 12V. Do this at several of them.
< 12 volts…Test voltage at the Glow Plug Controller and confirm 12V OUT.

No doubt you have a couple of things going on causing the Backfire (previously identifies as Poof) so lets confirm and correct what’s already known to be suspect.

Once that’s sorted out, and if the backfire continues, you can move forward. Its been two years so take it one step at a time.
 
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