Diesel Doctor

[Alex1944]

I have a '96 Chevy 6.5L turbodiesel, engine code "F", that starts, idles rough, but engine won't rev up. I have replaced the fuel filter, air filter, vacuum pump, and vacuum line from wastegate solinoid to wastegate valve. The only code that was set before replacing vacuum pump and line was P1656. I now have no codes but the truck still starts but won't run above idle. What should I look at next? I also checked the lift pump and it is okay. I have good fuel flow in fuel return line to fuel tank.

[JohnC]

Is the fuel clean?

Is there any air in the return line?

Was any major maintenance done that resulted in this problem, like pump replacement, timing chain, etc?

Otherwise, I think you're going to have to put a scanner on it.

[Alex1944]

Thanks, John for your response. There has been no major repairs just before this problem developed. The fuel looks clean and there doesn't seem to be any air in the fuel system. If I allow the engine to warm up to about 190 degree F. it idles very nicely and will rev up to about1600 to 1800 rpm. there is white smoke out the exhaust when I attempt to rev the engine. I have double checked the wastegate solinoid and it seems to work when I ground the terminal where the feed from the PCM is connected. I am beginning to think that the PCM has a problem. The PCM has a good ground.

[JohnC]

White smoke is unburnt fuel.
The wastegate is not the problem at this point.
Are there any codes stored?
My first guess would be that the injection pump is not getting enough fuel, either externally or internally.
when you changed the filter, was it bad? Water in the fuel? Algae?
Is there air in the return line?

What preceded the problem? Driving down the road? started next morning? Sat for 3 months...

[Alex1944]

The problem started several weeks ago as a very intermittent problem with the engine sometimes not reving up for 15 to 30 seconds after a normal startup. As time went on the engine required a longer warm up time before it would rev up. At that time the engine would run normally after it warmed up enough to rev normally. Now, after it warms up to 190 degrees F.,it may rev up to 1600 to 1800 rpm but that is all I get. It will no longer run good enough to drive. When I opened the drain valve to drain off any water, flow was slow but all that came out was clean fuel with no sign of water. The fuel filter didn't look extreemly dirty. The air filter needed changing, but wasn't that bad. The only code is "P1656". I didn't see any air coming from the return line. I may double check that. When it was running. I used the truck every week except for short vacations. Thanks again for your knowledgeable reply.

[JohnC]

p1656 is a wastegate code.

You're not running biodiesel, are you? Still sounds like a supply problem to me. Anything else would set a code.

Run the water drain into a jar and open it while the engine is idling. You should get a healthy flow, at least a quart in 30 seconds. (This eliminates everything up to but not including the filter).

[Alex1944]

Thanks, John. I'll give it a try this weekend. I wasn't sure if there is a problem with the PCM that I would get codes.

[Alex1944]

[quote="Alex1944"]Thanks, John. I'll give it a try this weekend. I wasn't sure if there is a problem with the PCM that I would get codes.[/quote]

Update: I pulled the fuel management filter assembly a to thuroughly clean it. I found quite a bit of solid debris in the bottom but almost no water. I checked the water drain line and it was almost completety blocked by this solid debris. I cleaned it out along with the water drain valve. I now have good flow from the valve. I pulled the fuel line from the injector pump inlet and checked for good flow and clean fuel. I then pulled the fuel inlet and found small metal shavings in the fine screen surounding the body of the inlet. I thuroughly cleaned this component. There is a small screen on the bottom of the component that I cleaned and blew air through. I couldn't determine if the passageway behind the screen was open and I don't know what the function of this screened passage is. I put everything back together, bled the filter housing again and started the engine. Since there was air in the fuel line from the filter to the injector pump, it didn,t surprise me that the starting and idle was rough. After a minute or less the idle smoothed out and the engine would once again rev to about 1900 rpm without hesitation. I am know back to the problem with the P1656 code showing up and I am not getting any vacuum from the wastegate solenoid to the wastegate acturator. I can manipulate the actuator with manual vacuum pump. The solenoid will operate if a ground is supplied directly. Any further suggestions? Thanks again for you help, John. I hope my experiences will benefit others.

[JohnC]

The wastegate issue is unrelated to the reving issue.

You need to get a scanner on it and see what the pump is doing relative to what the PCM wants it to do.
are you still getting white smoke? Maybe a compression check is in order. How many miles on the pump and injectors?


As far as the wastegate goes, the solenoid is pulsed to ground by the PCM. The duty cycle determines the amount of vacuum applied to the wastegate. At idle the pulse ratio (dwell) on the solenoid should be about 70% and the vacuum on the actuator about 15". First check the vacuum from the pump. If you dead end the pump to a gauge it should be at least 22". Cracked lines are a common problem, too.

[Alex1944]

Thanks again. I found the problem with the wastegate. The electrical connection to the solenoid was intermitent. I repaired this. I checked and repaired the vacuum lines when I replaced the vacuum pump.

My next move is to find a place with a Tech II scanner to diagnose the reminder of the problems. I think my problems are temperature related and don't understand why I am not getting codes.The injection pump was replaced under recall at about 90,000 to 95,000 miles. The injectors have been cleaned once at about 110,000 miles. The turbo was replaced at 130,000 miles. The truck now has 230,000 miles on it. I use fuel injector cleaner regularly. I am thinking about replacing the injectors because of their age and mileage. I'll let you know how things go. Working on this truck has become a hobby (and maybe a nightmare).