Engine/Transmission (1994 - 1998) Bucking Problem

[akline]

I have a 1995 reg cab 4wd 5 speed with 228,000 miles. I bought the truck two years ago, got a TDR membership soon after, and have been enjoying both. There is one small troublesome problem with the truck which I am about to explain, hoping that TDR will really prove its worth by helping me solve. The truck had a slight miss at idle to 1300 rpm, and above 2500 rpm would smoke clouds of white and flutter badly.

Against the advice of others and maybe my own better judgement, I bought it anyway. I'm glad I did, the truck has been great, but really want to get it running right.

A fuel filter helped the high rpm smoke problem, but it still does it somedays and sometimes runs fine(the weather?) Multiple fuel filters, testing the injectors twice, a new lift pump, checking the fuel precleaner, various fuel additives, a valve adjustment, and even backing off the aneroid and sliding the fuel plate wide open have not improved the first problem. (it did make a great running truck as far as power) My boost is about 25 and EGT about 1150 (before turbo) under load around 2400 rpm. My mechanic says I need to rebuild the fuel pump, but I want to be sure before I spend $2000!

The reason the miss/vibration at idle/low rpm is a problem, is that it transmits through the drivetrain under light throttle while gently cruising along, and quickly escalates into full scale bucking, which I'm sure would break something eventually. The odd thing is that under heavy throttle at any rpm it runs smooth as silk. (as only a Cummins will) It seems to me as though there would be some kind of broken spring in the governor or something. (I can do somewhat involved maintenance, etc. but major repairs are another story) I have been running the truck with the cat/muffler/ tailpipe removed since I was at a truck pull and wanted to make some noise. (don't tell my wife) With the more pronounced exhaust noise, I've noticed that when the missing is happening, it sounds as though the truck is running on one or two cylinders. The truck is still fine to drive if rpms are kept above 1500 under load, but I'm scared of what could happen if I let someone else drive it.

I've tried to be detailed so hopefully someone will recognize the problem, hopefully I didn't put you to sleep! Any help much appreciated!

 

[illflem]

Sounds like one of your throttle return springs is either broken or weak. Here's a description-

http://dodgeram.info/tsb/1997/18-29-97.htm



You don't need to follow the dealer's fix but rather pick up an equivalent spring at the auto parts for less than a few bucks.

 

[akline]

thanks

Thanks for the info. That's one I hadn't heard before. I guess I am a little skeptical, because it almost seems like the problem is on a certain cylinder (which almost convinces me I need to rebuild the pump) but it certainly is worth checking out. Thanks again

 

[Dave Remillard]

Piersdiesel.com



I would suggest maybe calling Piers if you can't get this resolved. There are several very knowledgable folks on this forum, but sometimes you need to go to the best. "IMHO" IT's only a few minutes and a toll free call away. Good luck

 

[Tim Galyean]

I have had the same problem for a couple of months now. I now believe that it is something in the pump, maybe the gov. springs.

I have checked motor mounts, compression, fuel pressure, throttle return springs. It still does it. What was described in the first post matches my truck exactlly. It used to pour out the black smoke, and since this problem developed it wont hardly smoke at all. Just about ready to pull the pump and send it in.

 

[HoleshotHolset]

... . this issue sounds like my how my truck is behaving.



I only encountered this when trying to boost launch or pull a sled-pulling sled. :-laf



This was after I installed some insane delivery valves.



Joe D. described the problem very well in a recent TDR magazine article. It's all in the governor section of the pump..... he explains it a lot better than I could.



Matt

 

[Tim Galyean]

I drove my buddys truck this weekend, and we found a stretch of open road, and really opened it up, and It started doing the exact same thing at full throttle, I asume it was running out of fuel, or coming up against the gov. I felt the same although mine happens at a cruising speed with light pedal.

 

[HoleshotHolset]

Mine only does the bucking when it's under a load. Nailing it while going down the highway doesn't make it buck.



With the delivery valves I have in there now - it bucks at idle.



Matt

 

[akline]

Which issue had the article that discussed the problem?

 

[cop0110]

My automatic was behaving like that and my lights would flicker very lightly in the dash. I took it to Dodge and they could not find a problem. After another two shop visits they replaced my crank sensor and it fixed the problem.

 

[HoleshotHolset]

akline: It was in the latest issue.....



Matt

 

[akline]

PROBLEM SOLVED

I am very glad to say that my TURBO DIESEL REGISTER membership has paid for itself 57 times over!!!!!!!!!!

In this past issue in one of the columns(I can't remember exactly where because my buddy has my issue) there was a section subtitled "Irv's truck dies". It spoke about an auto trans truck that would die when placed into gear. The author gave passing reference to a FUEL OVERFLOW VALVE, and it was like a light bulb came on. I pulled mine apart, found a broken spring, simply stretched it to compensate, and PARTY TIME! The problem is fixed. The bucking was caused by the pulsating action of the lift pump, since the overflow valve was simply bypassing most of the pressure to the return line, and every time the lift pump would stroke, the injection pump would get a 'shot' of fuel. I guess the reason it would go away under heavy throttle is that more suction was created by the higher volume of fuel being drawn by the injection pump.

Simply put, the 7100 injection pump is not designed to suck its own fuel from the tank, and a bad overflow valve is the equivalent of making it do so.

I saved approximately $2000 by not getting my pump rebuilt, so you do the math!

See the thread titled "white/blue smoke on a cold engine" or something like that under 2nd gen engine/trans to read my about my experience with the repair.

-I decided to cut and paste that here for those interested-



One thing I would check is the fuel overflow valve in the return line banjo fitting on the front passenger side corner of the fuel pump. This valve controls how much pressure is maintained inside the injection pump of what is delivered from the lift pump. If this valve is bypassing too much fuel it causes the injection pump to have to 'suck' the fuel out of the tank and it is not designed to do that-hence the reason for a lift pump. The smaller nut on the end will allow you to take the guts out of the valve-a small spring, a cradle for the ball(between the spring and ball), and a small ball. There also was a small washer that I found while working on this, I am not quite sure if it came from there or not. Right now it is not in there, if it was, I think it was under the spring where the spring sits on the bolt that holds it in the valve. These parts are easily lost(trust me, I know:mad I had to pull the guts out to get enough room to get it out between the injection pump and the intake manifold, then I could take out the banjo bolt, which contains the seat that the ball seals against. Mine had a broken spring, the problems caused were discussed in detail under 12 valve engine/trans: bucking problem. I simply stretched the spring to compensate for the broken part, I may get a new valve eventually(about $50). Use a 1/4" NPT tee fitting with an air guage on it, and use some teflon tape and very gently thread the assembled valve about a half turn into one side of the tee(the threads DO NOT match), and use a rubber tip blow gun to slowly apply air pressure to check what pressure the valve will hold and whether it leaks (you will probably get a bit of leakage where the valve threads into the tee-I want to figure out the thread size and tap out a tee for this purpose) I am told the pressure inside the pump is to be around 20 psi, I am not exactly sure what this translates to in air pressure while testing the valve, but mine is holding about 35 psi of air, and you could simply blow through it when I first took it off. I was around 25 psi, but that didn't completely fix my problem, so I stretched the spring a bit more. Hopefully my whole schpiel is helpful, I have spent TWO YEARS trying to find this problem on my truck, and am very happy to tell someone else what I found!