Do the '96 4.9L PCM's have a limp mode?

Mine has been down on power due to some undiagnosed condition for the last few months.

It's so sluggish that I'm afraid to try towing my trailer.

I dragged it into the welding gas place about 10 miles from my house to switch out a large nitrogen bottle, and it would barely get over highway overpasses in 4th gear at about 60mph.

I just had it checked over by my mechanic guy, who replaced the crank balancer because it had shifted (again), and we replaced the AC compressor because it seemed to be dragging the engine down when activated.

He also found that 4 of the 6 spark plugs that went in about a month ago failed in his spark plug tester. One would not go above 2k rpm, and others failed around 3k - 3500 rpm.

So now with the truly correct timing, new ac compressor, and new spark plugs that all tested good, it still runs just as sluggish as before.

What controls the timing in these engines? Or should I be looking at fuel pump pressure? Or something else?

I'm getting a confusing mix of CEL codes, consisting of excessive EGR flow, insufficient EGR flow, and DPFE high voltage (1401). These are occurring with the EGR blocked off at the intake manifold in an attempt to discover the problem (it runs worse with the EGR un-blocked).

And thoughts?

ADVthanksANCE

-Jack (sluggish in Central Texas)

 

Does it rev up sluggishly in neutral? Or only under load?
Fuel pressure should be the next thing to check. Along with the possibility of a clogged cat, use an infrared thermometer to check temps in front of, on, and behind the catalytic converter. The timing advance is controlled electronically, so just make sure that the PIP and SPOUT signals are reading properly, and that initial timing is set properly.

 

Thanks for the update, Shagg.

It revs up very slowly in neutral. When accelerating at partial throttle in 2nd or 3rd gear, it hunts like it wants to run, but something intermittent is holding it back.

The fuel pressure is good, 50 at idle, goes to 65 with vacuum hose to regulator removed.

I've taken the truck to the same shop that diagnosed my previous problem (no fuel pump activation with SPOUT in place), and they are testing the cats for blockage. Now, they are telling me that my cracked rear exhaust manifold might be the entire problem, but I told them that it has run correctly in the past with a cracked exhaust manifold. I'm going to ask them tomorrow if they have tested the PIP and SPOUT signals. They have indicated that they have seen erratic signals from the O2 sensors, and I'm wondering if that might be the real issue.

I'm pretty sure the timing is set correctly, as my regular mechanic guy (who is really knowledgeable and excellent) replaced the crank vibration damper and set the timing. This is the 3rd damper that's been replaced because of slippage of the outer ring where the timing marks are. Hopefully, this Dorman one will last.

I'm currently running out of ideas on this one. I've even tried a different PCM, as I still have a spare correct one after my previous PCM saga.

-Jack (mystified in central Texas)

 

So, the PIP and SPOUT signals tested good, as did my O2 sensors and my catalytic converters.

The shop is insisting that I replace the cracked exhaust manifold in order for them to proceed with further testing.

I know that it has run very well in the past with a similarly cracked exhaust manifold, so I am hesitant to continue with them.

The crack seals up after driving less than a half mile from stone cold.

Now, I have the truck back and it still runs very sluggishly.

I have found a local Ford dealer who is willing to give it a going over, but I consider that my absolute last resort.

Anybody got any ideas of what I can test next?

Thanks, y'all.

-Jack (can't outrun a Prius in my F150 in Central Texas)

 

Well an exhaust manifold crack will throw off the mixture that the O2 sensor reads, so it can definitely cause problems. You can try taking a look at the MAF and TPS. Did the shop that is looking at it look for vacuum leaks already?

 

My regular mechanic guy verified no vacuum leaks with his smoke tester.

Good call on the MAF and TPS. I'll definitely look into those.

Thanks.

 

No, Ford does not identify a "limp mode". Ford's 3 modes are normal, FMEM, & HLOS. This is cheaper, quicker, & more-effective than smoke:

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You should put ALL your truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

(click this text)


Put your location (nearest city) in your profile &/or signature, & upload an avatar of the truck. The more pics you post (NOT in your sig) of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you. Not all its details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now.

 

How do I determine if my PCM is operating in FMEM or HLOS mode?

Also, could operation in FMEM or HLOS modes be intermittent?

I still have to check on the TPS and the MAF sensor operation, but all of that is on hold right now as the hydraulic clutch slave cylinder has failed and the clutch pedal is on the floor.

Thanks to everyone for sharing their considerable expertise and experience.

-Jack (truckless in central Texas)

 

Does FMEM or HLOS mode apply to OBD-II models or only EEC-IV?

 

Yes, the EEC switches modes as-needed. There is no indication, other than the presence or absence of DTCs. EEC-IV & EEC-V(OBD2) use those 3 modes. Follow the abbreviations link on the VC label page (2nd thumbnail above) for more info.