Slight miss under power

[Tony J.]

I just noticed a slight miss in my '95 K3500 under hard acceleration (no trailer). I did notice this a few weeks ago when accelerating down an on-ramp while pulling my toyhauler. At that time, I had just been idling for awhile and assumed the FSD? was hot- it's a month old and mounted remotely on the firewall. Also, I JUST had my truck at the dealership last week, and they ran a complete diagnostic on it- no codes, nothing. They gave it a clean bill of health. I noticed the miss after they had looked at it, but I do know for sure that I noticed it before with the trailer, and dismissed it as the heat buildup from extended idle (90 degrees outside temp at the time).

The miss is very slight, but definitely noticeable. It seems to happen only when your foot's in it, and the rpms get up in the high 2000's. Normal town driving, never happens because I never rev it that high.

Anyone have any ideas?

Thanks!
Tony

[lupey6.5]

could be fuel starvation from the lift pump not getting voltage from the oil pressure sensor. while truck is running open fuel filter bleed valve. if fuel pushes out your OPS and lift pump are ok. if air sucks in your lift pump and OPS need to be checked. look in SEARCH for posts on further dignosis. if you can't find them post back. good luck

[rj]

I agree with Lupey. Sounds identical to symptoms of my 95 6.5 when the OPS failed.

[ucdavis]

Ditto on the fuel starvation cuz of occurrence @ high revs/foot in it.
But that's the high percentage guess; yours is whatever it actually is. Check your codes:
http://www.thedieselpage.com/members/trblcode.htm
for good measure (no code on a '95 for failing lift pump). Also if it is the LP, you may have an intermittent failure as the contacts oxidize up, so it might be running when you test. Best assurance is a new OPS (about $25 @ NAPA or other national parts houses) if yours is high mileage. Even if it seems OK but is suspect, fuel starvation seems to contribute to burning up the IP, so it is cheap insurance to make certain the LP circuit is A-OK.

[Tony J.]

UPDATE- well, I towed my utility trailer a bit today and definitely could feel the "miss" at about 60mph. Seemed most noticeable when I dropped out of OD and the revs hit about 2500. Very subtle, hardly noticeable, but definitely there. As I said, it also does it under hard acceleration with or without a load. I think I'll try the OPS as you guys suggested. Where's the thing located on the motor? I'm assuming something like that won't be difficult to change out. Will oil spew everywhere when I pull it? I've got a long trip in a month, so I'm thinking an oil change is in order before I go- that be a good time to swap the OPS?

Thanks!

[patrick m.]

the OPS is top/center, rear of engine. You may want to remove the fuel filter housing assy to gain better access. replace while engine is cool, because your hands will be all over it, and not much, if any oil will spill.
it will be easier if you purchase the socket made for the switch.

[gmctd]

Dirty fuel filter will give same symptoms, as will dirty air filter.
And low vacuum pump output can result in boost fluctuations.

[Tony J.]

UPDATE- Well, I just spend all day installing a bunch of mods, including the Heath chip and turbomaster. Right after I finished everything, with the truck idling, I opened the fuel bleed valve (?) on top of the engine. No fuel came out, in fact, the engine quit, and was a bear to restart. When I did get it started, it missed some for a couple of minutes, then it cleared up. I went and drove it, and when I put my foot in it, the engine was missing very noticeably, cutting out, in fact. I'm thinking this is my original problem (the miss I had in stock form), now made worse with the chip and the engine's increased demand for fuel compared to stock. Does anyone agree with this logic? I'm going to replace the OPS and see what that does. I'm still not quite understanding the whole LP/IP/OPS relationship...

Thanks in advance for any feedback!

[whatnot]

If you unplug the wires from the OPS and stick a jumper wire between the 2 outside pins, the lift pump should run. If it doesn't, it is a bad pump (or wiring problem.)

[N9Phil]

Just had the same thing happen to my 95. If I go above 2500 rpm it would miss. If I punched it while taking off and got above 2200 rpm it would kill. After stalling, I would put it in gear and turn the key to start. I would let the lift pump run for about 10 seconds and then put it back in park to restart. It would always crank a few seconds before starting. As long as I babied it, I had no problem. I ruled out the OPS because I always had good oil pressure and it never dropped when the engine would sputter. Two days ago I replaced the lift pump and problem is solved. Now when I take off and punch it, it doesn't miss a beat. smile.gif

N9Phil

[Tony J.]

Originally posted by N9Phil:
Two days ago I replaced the lift pump and problem is solved. Now when I take off and punch it, it doesn't miss a beat. smile.gif

N9Phil How expensive was the lift pump to replace? Did you do it yourself? How tough was the swap if you did?

Thanks for the help!

[Kollin Syverson]

Check the OPS first. I thought my lift pump was bad at first, so I put a new lift pump on, still nothing. I then put in a new OPS, and lift pump worked fine ( so did the old one ). I then cut in a relay so lift pump runs through relay instead of OPS. My truck ran just like you are explaining, would run fine until you put your foot in it. Now it never misses a beat.

[N9Phil]

I bought mine at CarQuest and the total cost with tax was $85.38. I would think that you should be able to find one for under $100.00. I installed it myself in around 20 minutes. It is not a difficult job at all.

N9Phil

[Tony J.]

YET ANOTHER UPDATE- Well, I replaced the OPS today. As soon as I did, I started the truck and shut it off again. After I shut it off, I heard something I've never heard since I've owned it- a fuel pump running for a few seconds after shutdown. Is this the "lift pump"? I'm headed out the door right now to drive it for the first time- I sure hope the high-rpm fuel starvation problem (assuming that's what it was) is taken care of now.

Keep your fingers crossed!

Thanks for all the help guys-
Tony

[Tony J.]

:D WOOOHOOOO!!! She runs like a raped ape! Wayyyyy better than before (chip), and no missing right on up through about 3,000 rpm when you stuff it! Thanks alot for your help guys! What did we ever do without the www? Let's hear it for Al Gore for inventing the internet!
tongue.gif :D

[Kollin Syverson]

Glad to here that was the problem. I would still put a relay in line, or it will happen again. The OPS just isn't heavy enough to carry the current of the lift pump.