Missing at Idle

[J DeMaio]

Hey guys:



The old Dodge seems to be missing or fluctuating in RPM while at an idle. It doesn't seem to run any different but noticed this the other day with the window down.



The truck has 176,900 miles on it. I'm pretty sure the injection pump has been rebuilt ( there is paint on the screws and it is shiny, not painted). I looked under the hood and found no fuel leaks, there is a little leak on the exhaust manifold but i think it is from teh valve covers.



The fuel filter was changed about 7,000 miles ago, it's a Baldwin filter. I only buy fuel from the "good stores" and the WIF light never comes on.



Any help would be appreciated.

 

[Archive]

J,

Sounds like a lift pump problem to me. I had the same problem as you have described, and the installation of a new lift pump cured it.

The lift pump provides fuel to the fuel injection pump under a constant pressure. At higher rpms, shortcomings in pump operation are less evident. At idle, however, the pump pressure (if worn) is insufficient for the engine to run smoothly.

The lift pump is located on the driver's side of the engine, about 12" forward of the firewall, and is mounted directly on the side of the engine block (as it is powered by the camshaft, I believe). Just follow the main fuel line aft from the injection pump. You can't miss it.

I think Cummins wants about $70. 00 for the pump and the installation is a do-it-yourself affair.

Hope this helps,

Steffan Marley

 

[Wayne M.]

J. You really need to diagnose this engine. First thing is to find out what cylinder is missing, and that is done by cracking each Injector line at the injector while the engine is idling. This will cause the fuel not to be injected into the cylinder and the cylinder will not fire. Each time a cylinder is killed, the RPM's will drop and the engine will get rougher. When you reach a cylinder that does not show an RPM change when killed, you have found the dead cylinder. At this point take note of the volume of fuel that is being squirted out. If no fuel comes out, suspect the Injection pump. If there is good fuel you can suspect the injector itself, but there is more to check. Next go to the valvetrain and remove the rocker cover for that cylinder and check valve lash. If all this is OK, then a compression test is in order.

A while back I had a Case Front end loader in the shop with a 6BT engine for power which had a dead miss. It turned out to be the #5 injector. I diagnosed it using the above method.

 

[J DeMaio]

Wayne:

To do this correctly, what do I need to do? I crack the injector lines to see if there is any change in RPM. Do I have to completely remove the lines or just loosen them a bit?



Also, I was looking around the lift pump area. There is oil/fuel on the block right around the pump. It looks to be the stock one and I don't believe that teh manual primer works anyway.



As for the valve train, I have gasket leaks so I will lash teh valves when teh leak gets bad enough, this could also be a problem, since I'll bet the valves haven't been adjusted since ti was out of warranty



What do you guys say for AutoZone fuel pumps and Fel-pro gaskets?

 

[Wayne M.]

J, Have you ever shorted Ignition wires on a Gas engine to kill a cylinder? If you have, then cracking lines will cause the same effect. I'm not positive of the rpm drop but you will definetly feel when a cylinder drops off and when it comes back after the nut is tightened. You want to just open the line nut @ the injector just a little bit as if you were bleeding brakes (maybe 1/4 turn). After you do the first one, you'll get a feel for it. I should reccomend shooting the nuts down with a penetrant and getting all of them to budge before you do the test.

One step I should have put in yesterday was to check the lift pump, as per Smarley. Judging from your mileage and the wetness you describe, it could be the problem. One way to check that is to crack the bleeder fiting and work the pump by the lever. You might have to bump the engine over to make the lever work. If the pump is "on cam" the lever won't compress the diaphgram since it already is.

I firmy believe in sticking with Cummins parts, especially gaskets.

 

[AlK]

J DeMaio, just a word of caution, remember the fuel lines going to the injectors are high pressure lines - so be careful so that you don't a get hurt.

 

[Scot]

I had/have a intermittant miss at idle when the tank got down to about 1/3 or less. It *appears* to have been a leak where the line comes from the tank to the lift pump. It was just damp enough to collect damp dust.



I am not sure if pulling it off and reseating it has fixed it or if I will have to replace the line or cut it and change ends.

 

[J DeMaio]

Guys:



I diagnosed the engine and the engine wasn't missing, it was surging. I did the injector check and they were all good, lines came off very easily. I haven't seen any more diesel fuel on the block, still looking.



Now for the question. I tried to prime the engine with the primer button on the lift pump on the engine and nothing happened. I just bumped the starter 3 seperate times and tried but there was no resistance, then I let it run a little, shut it off and tried again. Still a no go. I have tried to use the lift pump once before and the manual primer didn't work then, it was about 45* and I had air in the system, i ended up having to crank the Cummins over and over bleeding it by running the lift pump off of the starter, I know that wasn't good but that's all i could do.



Is there a way the lift pump will work and the manual primer won't



Thanks for all of the assistance.

 

[RamAZ]

Miss at idel.

I changed the lift pump on my 92 yesteday. I thought that it was having problems suppling fuel when the tank was down to 1/3. Comming up to that point shortly.



I went to Cummins and got the pump alot cheaper than the dealer. To tell you the truth it is a carter pump and some of the internet parts places have them. It is the same pump if it says carter on it.



I'll let you know how it runs later this week.



Jim

 

[Wayne M.]

J, seems like you just might have a bad lift pump. It's possible for the injector pump to do all the pulling. I was maintaining a perkins powered work boat a while back which didn't have a lift pump period. The owner didn't want to invest a $ either, so we had to make do, and it ran well. I would say to bar the engine over by hand (I use the alternator nut) and try the lever a couple times in 2 crank revolutions. If it's still limp, replace it. Look at it this way, if you ever run out of fuel on the road, you'll be screwed real bad without that little lever working!

Good luck.

 

[J DeMaio]

Thanks a lot guys:



I'll play with the lift pump a bit tomorrow and post a reply and tell you if it worked or not. Hopefully this is the problem. I noticed where there is RTV gasket maker behind the lift pump, between it and the block. I'll try cranking it over twice and then primering it.



Could out of "spec" valves cause this problem. I don't know if the valves have ever been adjusted before. The valve covers are leaking. I need a engine barring tool and some gaskets and I should do it for maintenance anyways. With 176,900 miles they would be out just a bit if they have never been adjusted.



Thanks



John

 

[Matt S]

The barring tool is a waste of money. Use a socket on the alternator nut instead. This makes it a one-person job finding the 'spot' forthe timing pin. The barring tool isn't easy to get too with a 4x4 and get the pin by yourself.

 

[Wayne M.]

John, Here are some reasons why the valvetrain can cause a dead cylinder;

1) The Valves are too tight (not enough lash), causing a valve to stay open and thus causing a compression loss. This is very unusual and in the case of a virgin / untouched engine, very unlikely. Could be a case of valve seat recession. This condition causes burned valves (and the main reason to check lash).



2) The valves are too loose (too much lash). If this would be found, there are problems to look for, like bent pushrods, loose rocker adjuster, or a wiped out cam or lifters. I actually did experience a wiped out cam/ lifter in a C8. 3 engine, in this case one intake valve did not open enough to allow enough air in for combustion, causing a miss.



Either way, checking lash is part of diagnosis and maintenence. Please consider using a 3/8 drive socket that fits the alternator (I think 7/8") and a long ratchet. Definetly the cheapest and easiest way to go. Get a feeler gauge and shop manual and go for it. In any case if you're not sure, don't loosen any adj. nuts, button it up and have a Diesel mechanic look at it, and maybe walk you through it. Good luck.

 

[J DeMaio]

Thanks for all of the help guys. I'm going to buy a new lift pump tomorrow. I'll get to the valves sometime soon and adjust them. I don't think it will be too hard Wayne. There used to be instructions on the old TDR website I wrote them down, I never checked my Haynes manual to see if it was in there, I bet it is. I'll let you know how the operations turn out.



Exciting time. I'm going to install gauges as soon as I get the time and I think I'm going to hurry up and buy one of Piers' "September Special". The 16cm turbine housing to get rid of that ridiculous turbo lag. Then I' ll have the Cummins power everyone dreams of

 

[J DeMaio]

Hey guys:



Thanks a lot for the replies. I bought a new Carter lift pump today and installed it in about 45 minutes. Boy, it's like new, the pump will prime the filter and it seems pretty tight.



It looked like there was RTV on the old gasket and the lift pump was a Lucas. It had been apart before. The primer on the old pump didn't work and the finger on the pump was very loose, I could move it up and down with my fingers.



"So far so good". No leaks and it seems to idle good. Thanks for all of the help.

 

[J DeMaio]

Well guys

 

[J DeMaio]

Sorry about that. After I changed my lift pump the other day I stilll noticed it was surging. I went and filled up with fuel and bought some Howe's winter treatment to put in. I think that helped It doesn't surge anymore. Could something have been stuck in the pump or could I have gotten a bad load of fuel



There's no doubt that the lift pump had already gone south. I could move the arm with my finger and the primer wouldn't work. It only took about 45 minutes to install and Now I'm comfortable changing fuel filters and If I run out of fuel I will be okay.



Thanks for all the help



John