EFI miss/stumble.....Fixed!!

[WorkerBee]

I am hoping someone can help me confirm my suspicion. I've had my motor going for awhile now but it starting to stumble and miss under heavy acceleration. I always had a slight hesitation for a second but not this bad. seems to get significantly worse as it reaches operation temp.

Explorer motor, GT40P heads, all stock. Mustang EFI, Ron Francis Harness, A9L computer. used MAF, BP, and Dizzy. Everything else is new.

I tested the usual suspects, fuel pump, fuel pressure regulator, TPS, charging system, plugs, wires, grounds, etc.

KOEO= 81,82, 84,85
KOER= 33,44,94

After reading about the problem sp71eb was having I am starting to think I have the wrong MAF. I bought it off an online mustang guy which said it was the correct one for my computer. Sensor part number is E9ZF-12B579-AA but I have always noticed the housing looks bigger than what it seems like it should be? Is it possible someone put the right sensor in the wrong housing? Where did this housing come from?

Here's some pictures of both. Thanks for any suggestions.

 

[sp71eb]

Damnit you have the exact same thing I had. Go to the parts store buy a rebuilt maf for a 95 mustang v6. Get a tamperproof torx bit (t20 I think) and take the sensor out of your housing and carefully put the new maf sensor in your housing. Unhook your battery for about 10 mins to reset your ecu. I let the truck idle for a few minutes after installing the new maf. Mine runs great now. I've put 60 miles on it this weekend and it runs great!

 

[Ourobos]

Check resistances on your coil.

 

[WorkerBee]

Damnit you have the exact same thing I had. Go to the parts store buy a rebuilt maf for a 95 mustang v6. Get a tamperproof torx bit (t20 I think) and take the sensor out of your housing and carefully put the new maf sensor in your housing. Unhook your battery for about 10 mins to reset your ecu. I let the truck idle for a few minutes after installing the new maf. Mine runs great now. I've put 60 miles on it this weekend and it runs great!

Man that what I feared. I am kind of hoping it's the same issue because I've tried just about everything. Just got back from pick n pull to try a junkyard unit. I'm going to go clean it up and give it a shot. Thanks!

 

[WorkerBee]

Unfortunately the correct MAF did not solve the problem. no better but no worse either. Anyone have any other ideas? I can try to ohm out the coil but heard that is often an unreliable test. If that's a bad assumption what should the primary and secondary coil read?

other things to look for are the TFI and dist. would a bad or weak TFI/Dist cause these symptoms? It runs great when cold. only minimal stumbling until it gets up to temp.

Thanks for any other ideas you might have.

 

[Ourobos]

I know I've seen a bad coil cause similar issues many times.

Normally a bad TFI will either not produce a spark, or will fizzle out once hot and then work when cold.

A bad PIP / Hall sensor in the distributor would likely cause a spark issue also, but it could create similar results as you're having if it's just on it's way out.

Your codes are mainly emissions other than the 84. Do you have the EGR vacuum solenoid hooked up (are you even running EGR)? A bad EGR circuit or lack of EGR can cause many problems, and that would be the FIRST place I'd look.

 

[sp71eb]

Did you make sure the sensor you got from the junkyard ended in A1A or A2A. Another thing is if the maf you got from the junkyard was also bad, did you disconnect the battery before restarting the truck. I have read some places that the computer can hold memory for over 1/2 an hour(not sure how true that is). I do know that at work if we have to unhook an airbag system on some cars we have to let it sit for 45 minutes with the battery unhooked before removing the airbags. One way or another the sensor you had before wasn't the correct sensor for that housing. My truck was running really lean under load with that sensor and I can't assume that to be good.

 

[Rox Crusher]

I was recently having same / similar issue. Mine would stumble under moderate load when warm. It felt like it was starving for fuel.

We did several things at once (cleaned MAF sensor wire, replaced plugs, cleaned IAB, cleaned throttle body, and cleaned air cleaner) which solved it for us.

I had all the normal EGR related codes plus 18 & 96.

I have driven it about 250 miles since fixing it, all the while not knowing what actually solved it and not being certain the fix is permanent.

Don't know if this info helps but I know how frustrating it can be so hope it helps some how.

 

[WorkerBee]

Your codes are mainly emissions other than the 84. Do you have the EGR vacuum solenoid hooked up (are you even running EGR)? A bad EGR circuit or lack of EGR can cause many problems, and that would be the FIRST place I'd look.

No I don't have EGR. According to my notes the 84 was present when I first got the motor running so hopefully I am not chasing an old problem. Also as far as I know the RJM clone harness already has provisions for omitting EGR. All of your other suggestions sound good. I'm going to do one more running fuel pressure test to see what's happening under load. After that I might just try a whole new distributor. Thanks Ourobos

Did you make sure the sensor you got from the junkyard ended in A1A or A2A. Another thing is if the maf you got from the junkyard was also bad, did you disconnect the battery before restarting the truck. I have read some places that the computer can hold memory for over 1/2 an hour(not sure how true that is). I do know that at work if we have to unhook an airbag system on some cars we have to let it sit for 45 minutes with the battery unhooked before removing the airbags. One way or another the sensor you had before wasn't the correct sensor for that housing. My truck was running really lean under load with that sensor and I can't assume that to be good.

Truth be told I bought two. One was a 55mm stock MAF. The numbers matched what I already had but with the smaller housing. I also bought an A2A in a 70mm housing out of a 94 Town Car. I'm going to try that one too. I figured if the stock one wouldn't do the trick the 70mm wouldn't.

I left the power disconnected for a few hours so I am pretty sure the computer was wiped. Thanks again for the help on this.

 

[Viperwolf1]

See that AFH55 number on the sensor? 55 refers to the mm size of the housing it's supposed to be bolted into.

 

[chuzie]

See that AFH55 number on the sensor? 55 refers to the mm size of the housing it's supposed to be bolted into.

I just learned something. ;D Thx.

 

[roderickl]

It sounds like one of the items mentioned in the other posts will have you covered, but I though I would still offer up a couple of things I have experienced, with similar symptoms:

1. Spark Related
   1. Bad plug wires
   2. Plug wires that became loose, but I could not tell until I took the wire completely off and put it back on
   3. Loose or faulty distributor cap
2. Fuel related
   1. Dirty injectors - I was not getting any codes, but the symptoms were there.

With the changes in fuel formulations over the last 10 years, this occurs more frequently.

When the cause has been dirty injectors, I run a can of Berryman B12 injector cleaner through my tank. A couple of years ago, I went through three iterations of my process of adding a can to 16 gallons of gas. Now, every couple of years I put a can in the tank. That seems to work for me. I find that Seafoam also works, but it is not as aggressive.

 

[WorkerBee]

Thanks for all the suggestions. I will keep digging and let you know if I find the culprit.

 

[ScanmanSteven]

Following your thread as I have recently had a similar problem. Not wanting to hijack, I'm learning along with others. I learned from Viperwolf1 that I shouldn't be using a 55mm MAF with my 65MM Explorer throttle body. So do I need to find a 65MM MAF then get a tune as this is on a 351?

 

[Viperwolf1]

I learned from Viperwolf1 that I shouldn't be using a 55mm MAF with my 65MM Explorer throttle body.

I never said that. I used that combo for a long time and it worked well.

What I said here is you can't use a MAF sensor in the wrong MAF housing.

 

[ScanmanSteven]

ok, misunderstood what you said. I now see it said housing, not throttle body.
Thanks for clarifying that.

 

[WorkerBee]

So I messed around all evening trying this and that. The only thing I found was the IAC seems to be stuck open. When I pull the connector the motor does not change at all. I removed it and it was pretty gunky. I emptied about a can of carb cleaner in it but it still looks open. After re-install the idle would hang a little longer than normal but I have not driven it yet.

so under throttle, does the IAC need to be closed? If so is it letting too much air past the throttle plate and leaning out?

Again I don't want to throw parts at it because I am actually learning a lot about troubleshooting but I think I might have to start.

 

[chuzie]

IAC will not have an effect on warm operation. The air is already metered by the MAF sensor and the TB regulates it. If the IAC is letting in additional air, it is already accounted for by the MAF sensor.

Keep in mind when you pull the IAC for testing, it may only make a difference on a cold engine.

You should be able to put some power to the IAC and hear the solenoid and see the plunger move. Actuation while cleaning will remove some of the gunk that may have a tight grip on the plunger shaft.

 

[broncodriver99]

IAC will not have an effect on warm operation. The air is already metered by the MAF sensor and the TB regulates it. If the IAC is letting in additional air, it is already accounted for by the MAF sensor.

I am not sure this is true. I believe the ECU looks at the TPS as well and calculates the mixture based on both inputs. The IAC being open would throw off that calculation.

 

[chuzie]

I have selectively extracted from the Probst manual so don't think all of this is from me.

The ECU only uses the TPS for feedback from the driver to determine strategy so you are correct in that it uses both the TPS and MAF sensor. I was dead wrong in my assumption. Here is a great opportunity to spread the wealth.

The TPS is used for:

1. Amount of throttle opening---how far is accelerator depressed?
Amount Is important to Cruise Strategy

2. Rate of throttle opening---how fast Is the accelerator
being depressed? Rate is important to acceleration
Strategy.

3. Closed-throttle position--ldle or deceleration.

4. Wide-Open Throttle position---acceleration enrichment,
A/C cutout, de-choke on crank..

5. Failure of MAF signal-TP helps control module to calculate
air Intake based on throttle opening and rpm.

6 Transmission-shift signals for electronic automatic transmission.

So yes, it does come into play when determining A/F mixture to some degree.

Now the IAC is driven by the EEC to:

1. Control idle speed according to a variety of engine
loads and conditions

2. Act as an electronic dashpot during deceleration,
preventing engine stalling. and preventing too-low
manifold pressure that causes excessive emissions.

3. Provides additional air during starting. bypassing the
closed throttle.

In normal engine idle operation, the valve is held partly
open allowing some air to bypass the throttle. It closes as necessary to reduce idle speed and opens to increase idle speed.

In dashpot mode, it allows bypass air to flow during deceleration to prevent engine stall. Of course. as the engine speed reduces, bypass air must be cut off to allow engine braking and to prevent fast Idle.

100% duty cycle is the normal setting for CRANK, providing what Ford calls "no-touch" starting.


What does all this mean? In my opinion, if the engine is above idle and not decelerating, the IAC will not be a factor for his issue. Now I do agree that if the IAC is allowing more air past the TB the A/F mixture will be modified to force the TB to close slightly to compensate? Yes. I misspoke in my previous post so thanks for calling me out on it and forcing me to educate myself. It will lean it out, but we are talking about drops in the proverbial bucket here. This would be a concern during idle only.

Will an open IAC cause stumbling or missing under a heavy load? No way.