Stumble, hiccup, what the...?

[Lightyear]

Here's the deal: I replaced the dirty nasty fuel filter with a new Stanadyne insert, bled the air out. Cleaned and re-oiled the K&N. New Johnson Controls batteries. Starts up and idles about a minute just fine. About 3 minutes drive through the hood @ 20mph, maybe 1200rpm. Coolant temp still showing 160, oil pressure about 60 psi cold. I pull out and get up to about 50mph, runs like a swiss watch. Back off the power to stop at an intersection, and it just...dies. Panel full of idiot lights with a stalled engine. Stop, neutral, turn the key she fires right back up. I pull onto the 2-lane on a slight uphill, and as I step on the loud pedal, it's go...hiccup...go...hiccup...go...hiccup, hiccup...go. No further stumbles on my 50 mile commute. I don't even know where to start looking. Any ideas out there?

[Robyn]

As it was fine for the rest of your trip I am suspect that your filter change just prior to heading out left some air trapped in the system that worked its way through.

It does not take much air to really make these run sour.

If this anomally comes back then its time to worry about it.

Likely you had an air bubble still in the system.

When bleeding the air out after a filter change its mandatory to do so with the engine shut off. Power the lift pump only and run it until all the air is out of the filter. Then open the drain valve up front and bleed a little more until that too runs free and clean.

Cranking the engine to bleed the system will introduce a lot of air into the IP and injectors.

Hope all is well

Missy Robyn

[Lightyear]

Well, it did run fine after that. The curious thing is that this behavior had happened before the filter change, also. When swapping the filter, I did bleed it with lift pump only, but I forgot about the drain valve up front.

I initially thought lift pump, but I can heat it running during glow cycle prior to start. Is there also a chance the PMD is getting finicky? The IP was replaced at about 40K under warranty, and the 'Burb has 106 K on it now.

I will bleed the water drain asap, and keep an eye and ear on things. I'll keep y'all posted. Thanks again!

[JohnC]

The drain up front is for water, not air. (Not part of the problem.)

Just because the lift pump runs when you turn the key on doesn't mean it runs reliably while the engine is running...

[Lightyear]

Zounds! I didn't consider that. Is there a way to test the lift pump? This problem is intermittent, usually occurring only after a cold or 'lukewarm' start. If I've lifted off the throttle it'll die, but if I've got my foot in it, it will hiccup, and stay running.

[JohnC]

Hook a vacuum/fuel pressure gauge to the water drain, open the valve, and go for a ride. If the pressure drops or goes negative when the problem occurs (or ever for that matter) you probably need a new oil pressure switch.

[DmaxMaverick]

I agree. Probably the lift pump, either failing, or not getting steady power when it should. Could also be a wicked air leak, somewhere....

[Lightyear]

Verrrrry interesting. I think I have a vacuum gauge, so I will try that out. Is it possible for the oil pressure gauge to indicate normal range and still be failing? On a cold start, I get up to 60 psi under load; warm, at idle, 20-25 psi; warm under load, 40-45 psi. Either way, I will probably replace both just to be sure.

[JohnC]

Yes. Two independent functions inside the same box.

[Lightyear]

I see, said the blind carpenter, as he picked up his hammer...and saw. Anyway, where is the oil pressure switch located on a 6.5? OE ones are cheap at O'Reilly, only 32 bones. Probably do that this weekend.

[JohnC]

I've never actually changed one on a later block, but I understand it is in the valley on the driver's side, near the center fore and aft.

[Lightyear]

Sounds like a hoot. Oh well, it needs to be done, so here I go...

[jasondmann]

To change the oil pressure switch unbolt the fuel filter/water separator from the intake but leave all hoses attached. There should be enough room to just move the filter assembly aside(towards the back of truck). It makes it easier if you use a crow's foot wrench.

[joed]

I thought the 96 and newer (OBD 2) ran the limp pump via the PCM, rather than the OPS.

Your oil pressure readings seem typical - I would verify it's pumping (or not) with the engine running before changing the OPS. You can open the water drain valve (slowly) by the oil filler tube while it's running to see if it's pumping. If you don't get fuel right away close it or you'll probably air lock the engine - I did this to my dad's 95 :eek:

My friends 98 just had a bad lift pump - it ran electrically but would not pump.

Hope this helps.

Joe.

[Lightyear]

The lift pump defintely pumps with the key on prior to start, because I got a bunch of fuel when I bled the filter after a change. I'll try the water drain, too. Already have an OPS on the way; it's only $30, and probably wouldn't hurt to replace it after 107K. Thanks for the tips, everyone!

[Lightyear]

Forgot to mention this...yesterday as I was heading for home, started in the parking lot fine. Idled about 45 seconds, long enough to put away the windshield sunscreen and it died. Started right back up, and ran okay all the 50 mile way home. So I will check vacuum at the water drain, check for fuel, and find a way to test the lift pump while the engine's running, right after a cold start. I feel like Sherlock holmes, looking for clues.

[Burning Oil]

You could have a pluged up tank sock also. Could also be a PMD problem?

When you do your OPS consider my extension hose. It will make the job alot easier next time. http://www.pmdcable.com/

[Lightyear]

I picked up an OPS last night, going to put it in this morning. Will definitely consider the extension as a future project. I don't think this whole thing is PMD-related, but that's just a guess based on what most folks have experienced and written about. It is a very intermittent problem, difficult to pin down a set of circumstances to make it happen. I just want to make sure it doesn't strand mama somewhere while I'm halfway around the world on business. So if I wind up r&r several things over time, that's alright with me. Also don't know if I mentioned the IP was replaced under warranty in 2004 or so, dodn't remember the exact timeframe. But any and all suggestions/tips/SWAGs are always appreciated!