Engine Surging idle only on start up

AeroCoupe said:

It can cause all kinds of weird problems.

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Interesting, I didn't know about that. Guess it wouldn't be too hard to take a look, although my car is a 1993 so you would figure that Ford would have fixed issues like that by then.

Mustang5L5 said:

With the engine running, Unplug the IAC. Does the engine stall?

If so, the throttle blade is too far closed. Need to open the set screw a bit and perform an idle relearn procedure. Sometimes when the IAC is being called on to 100% manage idle due to the blade being closed too much) you get a surge on startup. The car should be able to idle somewhat (although at a low rpm) on it's own with the IAC unplugged.

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I will try this as well.

General karthief said:

Vacuum leak
Low fuel pressure
air flow disturbed going through MAF
mis matched MAF/injectors
improper reset procedure
wrong/mismatched/worn parts
wiring issues
ignition switch

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The thing that is really throwing me for a loop is that the car behaves so well once it's done having it's little hissy fit upon start up. I feel like that would rule out something like a vacuum leak or fuel pressure problems. I will look anyway though. I also doubt that air flow to the MAF could be disturbed since it has a stock airbox and stock intake tubing.
I at least know that the MAF and injectors and any other electronic sensors are correct parts because it is all OEM equipment (or an OEM equivalent replacement).

Could it be O2 sensors? I will say that I have Bosch O2 sensors and they have always been a little problematic and I have been considering switching to Motorcraft ones. My impression on how they work though is that the computer doesn't take inputs from them until reaching operating temp so I don't know how they would affect the engine start.

conigz said:

I'm not sure if this has been asked yet. I did a quick browse of the comments. Do you have a stock computer with a 4 bank chip?

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I have a factory A9L and factory tune, it even still has the little "do not remove or it will void your warranty" sticker over the opening.

After further diagnosis I am pretty convinced that there is something wrong with the MAF circuit. I can unplug basically every sensor or combination of sensors and it doesn't change the starting condition except if I unplug the MAF, at which point the car starts and idles rough (as you would expect to happen when unplugging the MAF) but the engine is stable and does not surge. Since the MAF is new and I tested two different new sensors, I think I can conclude that the sensor itself is not the problem.

Now I am wondering if there are any common failure points in the MAF circuit that I should look for, such as the connector, somewhere a wire could become worn, or if the ECM could have issues. I will test all the wires but it's kind of a PITA so I was just hoping to be pointed in the right direction.

Alright finally got around to wrenching on the car some and here is the update.

I found about 1 ohm of resistance between the MAF signal return pins at the connector and the respective pins on the ECU connector, which I think is low enough to be fine.

I verified that the MAF power wire and the MAF signal return wires are an open circuit (infinite resistance).

This would suggest that the wiring is functioning properly, although it doesn't necessarily rule out some kind of bad connection between the male and female pins in the connectors themselves.

I also suspect that my old MAF was going out for quite some time before it became bad enough to trip a code, because I discovered that my high idle condition that started happening after I replaced my MAF was actually caused by a neutral sensing switch issue that didn't rear its ugly head until I had a properly functioning MAF. So I fixed that and my idle is now at a normal (for a cammed engine) 750RPM or so. One thing checked off at least.

The NSS working properly also fixed my engine surge on cold start. My theory is that now that the ECU doesn't always think that the transmission is in gear, it can actually enter the high idle procedure that is meant to warm up the catalytic converters when they're cold. The engine stays at about 1100RPM for maybe 30-45 seconds now before dropping down, which it never used to do. This provides enough RPM to override whatever else the computer is upset about.

The car will also still trip code 56 at WOT but only about 50% of the time and still struggles with surging hot starts.

I think I will try what you suggested @RotFox90 since a MAF connector is both really easy and really cheap to replace. Will update on results.

New connector made no difference. I am stumped. Maybe this is just how things are going to be with the modifications I have made to the engine? It's not too inconvenient but I am a bit of a perfectionist when it comes to this car. Fuel economy is normal so I presume that the MAF and oxygen sensors are doing their job at least most of the time.

Still open to any advice if anyone has ever encountered something like this and was able to fix it.

KRUISR said:

Have you done this yet?

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Yeah I have, it doesn't seem to make a difference.

93royalblue said:

ever figure your problem out? I’m experiencing something similar. Starts good when cold but when it’s warm and I start it it dies or surges really hard until it finds idle. Then once u get to driving it it drives perfectly fine. The only problem is start up.
I’d check your ground and make sure to run at least a 4awg feound to body if you have an upgraded alternator. That helped mine out a bit

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I'm in the process of having my ECU repaired to see if that fixes my issue. Haven't gotten it back yet but the technician did tell me that the computer was malfunctioning and needed various repairs to restore normal behavior.

I'll come back with updates once I can actually test it out.