97 5.0-Low idle and stumbling...Help..

[SVO LOU]

Guys,
Have a 97 Mounty w/82k. Went to go out last night but it was hard to start. When it finally did, it was idling really low, probably less than 600rpm and sounded like it had a miss. Tried to rev it in park and it hardly got over 1000 rpm and it backfired through the intake sometimes. It defintely won't move and no CEL/codes.

Today it had the same symptoms BUT if I unplugged the MAF sensor it would rev to 4-5K no problem and I couldn't detect a miss. Fuel pressure was at 35 psi at idle and all wires have a spark.

After that I changed the plugs, fuel filter, cleaned the MAF and IAC valve, checked (unplugging/replugging) the coil pack connectors and 1 O2 sensor conn., and disconnected the battery for 20 minutes. I changed the plugs and wires about a year and half ago, wires still "looked" good (no obvious burn marks).

Here's the kicker, in my eyes anyway...This just happened over night (literally). Wires usually offer decreasing performance. Fuel pump and sensors usually go out like this BUT I have fuel pressure and no CEL.

Any ideas?
Thanks.
Lou.

 

[exploded99]

My 99 V8 calls for 65 psi of fuel pressure + - 8 psi. Isn't yours the same?

But it sounds like the MAF is not reporting to the PCM correctly. When you unplug the MAF, the PCM runs the engine off of tables in the PCM and relies heavily on the TPS if I understand it correctly.

Anyway you can get a real time scanner hooked up to see what the MAF is doing?

 

[SVO LOU]

My 99 V8 calls for 65 psi of fuel pressure + - 8 psi. Isn't yours the same?

But it sounds like the MAF is not reporting to the PCM correctly. When you unplug the MAF, the PCM runs the engine off of tables in the PCM and relies heavily on the TPS if I understand it correctly.

Anyway you can get a real time scanner hooked up to see what the MAF is doing?

Wow! It's really supposed to be that high? I used to be an ASE Ford mechanic about 10 years and About 38 @ idle was plenty. My, how things change.
I'll look deeper into fuel delivery.
Thanks.
Lou

 

[exploded99]

Well, 65 is what alldata says. I have seen typos there.

Its probably Fords way of increasing fuel delivery at high rpms without spending money for bigger injectors. I would guess you get a very nice roman candle effect when the fuel line breaks.

 

[SVO LOU]

Well, 65 is what alldata says. I have seen typos there.

Its probably Fords way of increasing fuel delivery at high rpms without spending money for bigger injectors. I would guess you get a very nice roman candle effect when the fuel line breaks.

The little info I've found so far says close to 40 is fine. I drove it with the MAF unplugged so it'll get me to work and back but drive-ability isn't great. With the MAF plugged in I'm lucky to get it to 1200 rpms. And that's feathering the throttle!

Still open to suggestions from anyone.
Lou.

 

[exploded99]

Ok, guel injection system is different on the two years.

Still think its your MAF - need to check it.

 

[SVO LOU]

Yep, pretty weird. My first thought is the MAF also, but it doesn't throw any codes with the MAF plugged in. I get 0102 with the MAF unplugged BUT I'm able to drive it. Thought it was bad gas but I emptied all but 1/4 of the gas and filled it back up. Same thing. Definitely stumped here.
Think I'll plug in a MAF from my Mustang to see if anything changes.
Lou.

 

[exploded99]

Well, I wish there was a writeup somewhere on what the pcm does when you disconnect the MAF.

Other thought would be the camshaft position sensor cause you said you had a backfire and the engine uses that for timing. Don't think you get codes there either. Don't know if that gets bypassed when you disconnect the MAF and the PCM goes to tables? I have not heard of anyone complaining of backfires thru the intake with a bad MAF, and your MAF is not setting a code, so it might be worth a look see.

Since it does not run right off idle the IAC is probably not a factor in the idle problem -

 

[SVO LOU]

Yep, still weird. I pulled the CPS and it "visually" looked perfect. Others have said theirs were cracked or corroded. I guess Crankshaft Position Sensor needs to be looked at next.

I'm really into desperation territory because I'm thinking of dropping it off at the dealer. I used to be a dealer tech so you know I'm scraping the bottom of the barrel. Just frustrating not having a scanner to "see" real-time voltages.

Lou.

 

[exploded99]

Have you checked the EGR valve to be sure its not open at idle with the maf plugged in? Again, not sure but the EGR could get disabled when you pull the MAF connector and the engine goes into limp mode.

Perhaps just pull the vacuum line off it and see if driveability and idle improves with MAF connected.

Can't hurt to check the Crank sensor. I have heard of the connectors on those getting corroded. My understanding is the engine gets a reference pulse for TDC off the crank sensor when starting, then uses the Cam position sensor for timing.

Long shot, but the a/c clutch is cycling? Not stuck on? Maybe even pop the belt off and see what happens. Nevermind, I was thinking about the idle only -

It can't be coil packs, plugs or wires cause it revs fine with the MAF disconnected. Those parts are not bypassed by disconnecting the MAF. Can't be internal engine issue or, again, would not rev good just by disconnecting MAF. Fuel pressure is good - can't see it being a fuel issue - and again the MAF experience rules it out. Water in gas could not just "go away". CMP seems good. TPS works good when MAF is disconnected and engine revs up. So you are left with perhaps an engine temp sensor doing something weird. Its beginning to narrow down to the MAF, but without a real time scanner to check out things its hard to be 100%

Well, post back and let me know what happens. My idea tank is running dry.

Just a thought, but with all those Fords I would think you could justify a real time scanner! Two dealer visits (diagnostic fees and labor) and you're just about covered. However, the better half often controls the "toy" budget.

 

[Billy177]

you have a bad maf

 

[performancenut]

you have a bad maf

Pull a code. Bet it will read for a MAF issue too.

 

[Billy177]

it actually most of the time a bad maf will not throw maf codes. i bet the only codes related to it will be just from running the engine with it unplugged

 

[410Fortune]

Right, you could confirm this by monitoring the MAS in real time, and good OBD-II scanner can do this, like the Snap On unit, many shops have these suckers and can run a KOEO and KOER test for ya less then the dealer.

OR

You can swap the MAS with a friends real quick, reset computer, drive it and see what happens.

I am sure you can also monitor the MAS output to the PCM in real time with a multimeter, but I too am guessing you have a bad MAS, and since it changes when you unplug it and still get no code the PCM is suspect as well.

Let us know what turns up.

 

[SVO LOU]

Pull a code. Bet it will read for a MAF issue too.

it actually most of the time a bad maf will not throw maf codes. i bet the only codes related to it will be just from running the engine with it unplugged

As stated before, no codes with the MAF plugged in but I get 0102 with it un-plugged.

Took it to my friend that works at a L/M dealer. He first found the front/pass-side O2 sensor switching very slowly. Maybe I carboned it driving there with the maf un-plugged. Maybe it was just due, who knows. He tried a new one and then drove it, and he said it ran great, but then it started doing that very low idle thing again.

He then found that the 5 volts that goes to the maf sensor (VREF) to drop to .8 volts, intermittently. Bingo! The only problem is that he can't find out why yet. We're getting there, though.
Lou.

 

[exploded99]

I was beginning to think SVO LOU had left the building......

Thanks for posting that - I was curious. I really hate those intermittent electrical issues. They are hard to find and mimic other problems, so you end up chasing deadends.

Hope its back to normal soon!

 

[SVO LOU]

I was beginning to think SVO LOU had left the building......

Thanks for posting that - I was curious. I really hate those intermittent electrical issues. They are hard to find and mimic other problems, so you end up chasing deadends.

Hope its back to normal soon!

Haven't posted in some time and wanted to update...
Turned out that the ECM was the only thing left so we tried it and, voila, runs like a charm. My friend did find a bad ground to the MAF so we're thinking that the ground fried the MAF which, in turn, fried the ECM. Maybe the ground fried the ECM which fried the MAF. Either way, good to go now.
Lou.