Misfire when pulling

[fastcat800]

I have a '94 6.5TD K1500 with deep gears (2500 RPM at 70 mph). When pulling a 4000lb trailer the engine starts to misfire when it gets to higher RPM in 3rd gear. If you back off the throttle to half it clears up in a few seconds. It starts and runs great. When not pulling a load if you accelerate hard three or four times in a row it will start to misfire too. The truck has 180K miles with 30K on a new engine and has had the turbo replaced too. This is all I know as I bought it from a friend who got it after his dad died. I have replaced the fuel filter and have checked for error codes. The only code I get is vacumm related. There is a very small crack in the intake manifold. I plan on replacing it soon but if the injector pump needs replacing I would like to them both at the same time. The PMD looks newer and has the latest part number on it, 583 I believe. Could this be lift pump related? I would think if it was related to vacumm it would misfire all the time. It has to be a fuel problem I believe. Any ideas?

[Cowracer]

Seeing how it misfires when fuel demand is greatest, I would bet its fuel starvation. Look for (in no particular order)

</font> Clogged fuel filter (new filters can clog too)</font> inop lift pump</font> deterioration in fuel lines allowing air leaks</font> poor fuel quality</font> water in fuel tank</font> clogged intake strainer in pump inlet</font>

Hope this helps

Tim

[BUZZ]

DAve,
Sounds like a lift pump. the lift pump is powered by the oil press switch. Located rear of intake next to fire wall. Lots of oil press switch failures. Crank your truck and crawl under driver door and locate lift pump in frame rail. You sould feel it thump away. You can trouble shoot from there.
Buzz

[fastcat800]

How do I test the lift pump? Buzz, are you saying it could be a bad oil press switch as well?

[fastcat800]

I just looked at the lift pump and it looks brand new. I was suprised at how small it is. About the size of an inline fuel filter with wires running to it. I don't see anything that looks like a strainer.

[fastcat800]

Is the strainer near the lift pump?

[Cowracer]

the strainer is on the front of the injection pump. It is (as you are facing the engine) just to the right of shutoff solenoid. It is where the rubber fuel line comes in to the pump. Remove the line, the hose barb fitting and then the strainer. You may have to remove the shutoff solenoid for clearance. simply unplug the connecter, and unscrew the solenoid.

Tim

[fastcat800]

Thanks Tim, I will try that tommorow.

[fastcat800]

The strainer was spotless. Anyone else have any ideas?

[patrick m.]

Dave, you didnt mention if the lift pump was working or not.
also, if you can feel it pumping at idle, that doesnt mean it is pumping at cruise speed.

best thing to do is replace the oil psi switch, which is what powers the pump after start up.

[joed]

Another way to verify the lift pump & OPS are working correctly is to temporarily plumb in a fuel pressure gauge while driving.

I just did this on my truck - I have a combo vaccum pressure gauge that I hooked to the water drain T at the front of the engine. For temporary, testing, purposes, I placed the gauge under the windshield wiper arm so I could read fuel pressure fluctuations while driving. Obviously you have to have access to a local, low or no traffic type road to do this safely (should do full throttle accel. tests to see how much press. drops). (Not to mention trying to explain what you are doing to a cop, should you get pulled over)... ;)

Anyway, hope that helps. Joe.

[fastcat800]

I am not sure the lift pump works but it looks brand new. What pressure is normal for a good lift pump? Joe, I am confused as to where you teed into the fuel line. Couldn't you install a tee where the line goes into the strainer?

[joed]

Dave,

Actually, it's really not a tee per se - since my gauge already has vacuum type hose with it, I just pull the rubber hose that comes off the water drain "T" valve at the front of the engine near the oil filler cap and push the gauge hose onto the barb (it's the horz. hose, not the one coming from the bottom). Open the T valve while the engine is idling and you should see 4 psi, min. If that checks okay, monitoring under load (driving) would be the next step to see if the pressure drops significantly.

Hope that helps. Joe.

[fastcat800]

Now my water in the fuel light is comming on intermitently. I have about a 1/4 of a tank of fuel and I added about 8 oz. of cetane and drove it for about 2 miles and let it idle for a half hour. The light is still coming on. It still runs the same. Maybe this has been the problem all along. What should I do next? I need to get this fixed as Thursday I am leaving on a trip to northern Wisconson.

[JohnC]

You cannot/don't want to send that water through the system. Diesel fuel systems are designed to separate the water out. That valve on the thermostat housing is to drain the water from the filter/separater. Pull the hose up and put it into a jar. Run the engine or at least the lift pump. Open the valve and drain the water out. Refer to the owner's manual for more information.

[fastcat800]

I tried to drain the water and when I open the valve I get nothing until about four or five turns and then the truck dies (no water or fuel out the line). Must not be any pressure. I have to crank for about a minute to get it started and it idles very rough now. Is the lift pump the thing that looks like the inline filter with wires running to it? When does it run on the '94's? When you turn the key on? Is it continuous or is there a pressure switch that turns it off and on? Any tips on replacement? I just filled the tank with fuel.

[JohnC]

You have just performed the crudest lift pump test. It failed. The lift pump should run when the engine is cranking and when the engine has oil pressure. Have someone crank the engine while you try to drain the water. If fuel comes out then but not while the engine is running, the oil pressure switch has likely failed, a very common problem. If neither condition produces fuel or if the pump runs but does not pump fuel then the pump is likely at fault.

Fix this and your miss fire will probably go away too.

Here's a hot tip to keep the oil pressure switch from failing: Oil switch fix (http://www.thedieselpage.com/features/sutsos.htm)

[fastcat800]

Now it won't start at all. I have a new pump. I am going to test to see if there is power to the pump when cranking before I replace it.

[Hubert]

If you don't want to drain the tank then at least add some water dispersal additive like PS diesel 911. At 1/4 tank I would recommend draining it. Water robs diesel of its lubricity and will hurt your IP. If you use diesel 911 use the right amount its not good to overdose it. Also add a good lubricator like Lucas. Cetane additives only increases the "power in the fuel".

I just changed my lift pump this past weekend. It was ~ $95 from Advanced Auto. Mines a 97 K2500 ext cab.

Its messy but easy. I think it was a 5/8 wrench and 3/4 wrench (to hold the pump). Unbolt the little bracket top and bottom unplug power then undo the lines. They are going to drain fuel so have rags ready and new pump ready to install.

I like to spray a penetrant (wd-40 or similar) on flare nut connections first to help break the nut from the line in case its corroded and keep the flare nut from gualling the line. It takes just a few minutes to do.

Might as well change filter now too while the filter bowl is drained. Clean the bowl real good. Maybe add a little lucas to the filter bowl or wet the filter element. Careful not to fill over the stem (exit to the IP). The filter needs to be fully installed before filling the bowl full.

Let the new pump fill the bowl like regular purging the air and let it drain out the water seperator too a little extra for any inline air.

I like lucas brand. Its pretty good stuff and not harmful if overused some. Some additives recommend at least 50% diesel if you put it in filter bowl.

For the next few tanks add lubricators additives and change the filter again soon and see how it looks.

Good luck

[fastcat800]

Just got done changing the pump and filter. The water light was out earlier today when I was driving so I filled the tank. I checked for power at the pump when running and have 12 volts so the oil pressure switch is OK. I can feel it pumping when cranking but had a hard time telling when it was running so I checked for power to be sure. After I changed the pump and before replacing the filter (the one in there looked new) I tried to bleed of any water and the same thing happened. I opened the valve and nothing came out and it quit running. I am afraid to open the valve again as know it will quit again. What could there be between the lift pump and the injection pump besides the fuel filter that can restrict fuel flow?

[DKeen]

I have been having the same problems. Here is what I have done, without sucess on fixing the problem.

New lift pump
new filter
new electronic filter harness
new fuel tank (plastic this time)
Fuel relay
New pmd
New OPS

This thing is driving me nuts. The stumble happens infrequently and only when you are requiring a lot of fuel. I will take any suggestions- HELP!

[Hye]

Dave,
Maybe there is a clog between the pump and fuel in the tank.
Did fuel run out of the line from the tank when you changed the lift pump?
Or, does it now pump with the line removed at the pump outlet?
Or, have you tried to run the pump you took off the truck?
Keep us posted.

[JohnC]

Check the power to the lift pump while the pump is connected. 12 volts into an open circuit does not guaranty the switch is good. Also, check the pump output while cranking as this bypasses the oil pressure switch.

Also, if you apply 12 volts to the upper left contact (red wire) in the diagnostic port under the dash it will power the pump directly.

[ 02-09-2005, 08:13 AM: Message edited by: JohnC ]

[fastcat800]

I am headed to the parts store to buy a OPS right now. Any tips on installing it? I a not sure where it is on the rear of the engine. The lift pump was only $81 at Carquest. I had fuel runnning out when I changed the pump.

[fastcat800]

OK where's the switch? I don't see it. I'll bet it's under something on the rear of the engine. Lots of connectors and wires back there. I don't have any deep sockets big enough to fit it as it's over 1 inch hex. Can you get on it with a 10" cresent wrench to get it out or do I need to go buy a deep socket?

[DBlake]

Dave,
Just take off the back connector and slip a special deep socket over it. Autozone has'em for about $6-7. Its called a GM OPS removal tool I think. Much eaiser than the wrench route.

[fastcat800]

Now I see it. The old one is going to stay in for now. I don't have time to mess with it before I leave. The truck does run better now but it still misses. It looks like the fuel filter housing needs to come out to get the OPS out. Even if I could get on it with a socket and a universal joint I dont think I an get my hand down there to get the new one started. What is the easiest thing to remove to get more room to get in there?

[patrick m.]

when you remove the fuel filter housing, the cluster of large connectors will be loose too, as they are retained by the same bolts.
Once the filter housing is out of there, and the connectors can be manipulated, you will have much better access to the ops.
I would replace it before my trip. making the IP "pull" fuel through the entire system cant be good for it.

It will only take an hour or two, much better than the alternative.

[fastcat800]

I changed the OPS today after buying a socket for $9 and it runs perfect now. I can even open the bleeder valve and get fuel and it keeps running. Haven't pulled my trailer yet but I am sure that was the problem since I now have fuel pressure. I am going to buy a relay and do the OPS fix so I don't have this problem again. I would already have it done but or local Radio Shack did not have a relay. I will have to go to Mpls to get parts. Only took me 10 minutes to change the switch. Thank God that was the fix.