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

[sp71eb]

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.

Would be nice if you were closer, you could try all of the new crap that I bought to try to figure out what my miss was. I have a new IAC, new TFI, fuel pressure gauge, used tps, used map sensor, and a lot of head scratching.;D. Did you try the other 70mm maf you purchased? I am super impressed how well my truck is running after swapping in the correct maf. I drove it to work Monday and it was -14 out, thing ran great even with the crazy temperatures. And yes it was a f-in cold ride.

 

[chuzie]

I am curious what your TPS is reading engine off. Probe the harness and get the voltage reading for us. I wonder if you are sending bad data to the EEC and it just can't compensate for out of range values to such a degree.

 

[WorkerBee]

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.

Wow. Learned a lot there. Thanks chuzie

Well I feel like a jackass. I think I have found the culprit though, buried underneath my "heat shield" socks.....burned spark plug boots. Don't ask me why I didn't pull them off earlier. I guess I was stuck on the fact that it started to stumble the very second it reached 190 degrees. when cold it runs great.

I hope I'm right here and will be ordering a set of ceramic boot plug wires to try. They're expensive I know but it's better than buying headers and redoing my exhaust. I have stock iron manifolds now.

If I have to buy some GT40p specific headers I will but would like to try the wires first. Damn gt40p heads. I knew they were going to bite me some day.

I'll report back if the new wires work. Thanks everyone.

 

[sp71eb]

Wow. Learned a lot there. Thanks chuzie

Well I feel like a jackass. I think I have found the culprit though, buried underneath my "heat shield" socks.....burned spark plug boots. Don't ask me why I didn't pull them off earlier. I guess I was stuck on the fact that it started to stumble the very second it reached 190 degrees. when cold it runs great.

I hope I'm right here and will be ordering a set of ceramic boot plug wires to try. They're expensive I know but it's better than buying headers and redoing my exhaust. I have stock iron manifolds now.

If I have to buy some GT40p specific headers I will but would like to try the wires first. Damn gt40p heads. I knew they were going to bite me some day.

I'll report back if the new wires work. Thanks everyone.

How many miles do you have on the plug wires? I have a similar setup. I have stock manifolds that I clearance for the gt40p heads with summit racing 45 degree plug wires and some fiberglass spark plug boots. Guess I would like to know how long to expect this setup to last. Hope you figured out your problem!

 

[WorkerBee]

I only have about 500 miles on them. They're the ford racing 45 degree boots from summit. I have the same fiberglass socks. I thought I ground enough off the manifolds for the plugs but I guess not. I didn't want to thin them down too much. I have a few pictures on my computer at work so if you'd like I can send them to you to compare. I really hope ceramic boot wires solve the problem. Thanks for the help.

 

[WorkerBee]

Problem solved. Runs like a champ again. These ceramic boot plug wires did the trick. Not cheap but I doubt I can melt the ceramic. That is until I turn up the turbo entabulator. Ha ha

Thanks for all the help on this.

 

[sp71eb]

Just wondering how the plug wires are working. I have got about 650 miles on my bronco and am starting to hear a slight miss. I have a feeling that I have a burnt plug wire. I am running summit 45 degree street strip wires with fiberglass boots over them. I was hoping that that would be enough but looks like I will be getting some 130 dollar spark plug wires also.

 

[Viperwolf1]

Those P heads will really put your plug boots to the test.

 

[WorkerBee]

Just wondering how the plug wires are working. I have got about 650 miles on my bronco and am starting to hear a slight miss. I have a feeling that I have a burnt plug wire. I am running summit 45 degree street strip wires with fiberglass boots over them. I was hoping that that would be enough but looks like I will be getting some 130 dollar spark plug wires also.

Sorry just saw this. So far so good. I don't have many miles on them but it runs great. The new wires don't fit nearly as well as the ford racing wires but I can live with that as long as it runs well. The wires aren't labeled so it took a little trial and error to get the best fit. I thought that was kind of poor, even a removable paper tag would have been nice.

If your starts running progressively worse I bet you have the same thing happening. I would remove the socks and run it in the dark to see if you can see them arcing to your manifolds. If I had done that earlier I may have caught the problem faster. Good luck. I hope you find it.

 

[Rox Crusher]

Problem solved. Runs like a champ again. These ceramic boot plug wires did the trick. Not cheap but I doubt I can melt the ceramic. That is until I turn up the turbo entabulator. Ha ha

Thanks for all the help on this.

That looks eerily familiar.

 

[67EB_in_619]

Accel makes a shorty plug that can help some too... and the nomex boots are good for radiant heat, not as good for direct contact. I had bought some direct contact heat shield designed for wiring but cant find the source to link... just make sure the glass boot cover you get is actually rated for direct contact heat.


http://www.summitracing.com/search/...atch&SortOrder=Ascending&keyword=shorty plugs

 

[roundhouse]

We used to have a P chassis Chevy bus with the 454

The manifolds would get so hot it would literally melt the wires

We wound up using the metallic flexible wire shields

My EFI EB eats wires as well. But doesn't seem to be heat related

They just stop working