Cold start bucking - 94 5.0 GT

snrusnak

Member
Oct 11, 2020
7
1
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33
Gulfport, MS
Hello, I'm working on a 1994 5.0 Mustang GT 5 speed manual. It's all stock with just a few modifications. I know little of it's history, but it appears to have had a somewhat recent tuneup just by looking at the cleanliness of the distributor cap and wires. It has EGR and AIR systems deleted, and exhaust work with no cats. The EGR pipe to the exhaust has a cap and welded on, and it has a blockoff plate on the intake manifold. It does NOT have the plug in thing to the EGR harness to trick the computer, and is of course throwing codes for it. It had a charging issue that I have resolved (broken harness connector at voltage regulator). It also had a rough idle, I have done the following: Cleaned the MAF, IAT, IAC (which appears to be recently replaced also), and also cleaned the TB. I went through the whole procedure of setting base idle (whoever worked on it prior completely backed the set screw out), I did the IAC relearn, then set TPS voltage to ~0.98 volts. After all that, the car idles and runs awesome, but only when fully warmed up. On a cold start, it has a rough idle (idle fluctuates a few hundred rpms), and if you try to drive it it bucks and surges like crazy. It's essentially undriveable until fully warmed up, then runs almost like new.

So, would the computer thinking there's still EGR cause this issue? I don't know enough about these cars and the computer logic to know. I am tempted to buy the plug in to trick the computer just to see, plus eliminate the codes.

Any other ideas? I will list all the codes it's throwing below, but none seem to be related to any of this issue in my opinion. I also did the cylinder balance test or whatever it's called and it flashed the CEL 9 times, which I believe means it passed with no issues/codes.

This is my first experience with obd1 which has been a bit of a learning curve to get the codes lol. I'm used to working on obd2 and mainly GM/LS engines. Would love some helpful hints here, I've about exhaust everything I know to do.

Here are my ideas before I give up:
- Wrong spark plugs? (heat range way too cold, causing it to load up when cold)
- Unplug IAC during a cold start and see how it idles/runs. If much better, try replacing the IAC. Maybe it's not working properly only when cold? (doubt it...)
- Try the EGR plug tricker thing. Maybe this is messing up the fueling/tuning when cold? Maybe!?!?
- I've read about the ECT sensor causing issues like this. It doesn't appear to be bad, the coolant temp rises as expected slowly and steady. But maybe....?

One other symptom, which may be normal, I'm not sure, as I don't drive this car regularly. When shifting gears, the rpms actually jump up a couple hundred rpms when letting off the gas and pushing the clutch in. Seems odd, like the IAC is opening up when shifting. The rpms will fall again, just seems odd. Maybe this is normal???

That's about all I got, lol, any help appreciated!!! Below are the codes:

KOEO Codes:
327 - DPFE or EVP circuit below minimum voltage of 0.2 volts (I assume this is due to the EGR/AIR delete and may go away with the plug in tricker thingy)
564 - Electro-drive Fan circuit failure (no idea why this is triggering, when the car does the self test I can hear it run low and high speeds. Not too worried about this one...)

Continuous Memory Codes:
212 - Loss of IDM input to EEC or SPOUT circuit grounded (I had pulled the SPOUT thing to check static timing, this may be why this one was set. Also, I pulled codes multiple times prior to checking timing and this is the first time it's set. So not worried about this one right now, I'll clear them next time I pull codes...)
327 - Same as above

Engine Running Codes:
314 - Thermactor air system inoperative - left side (I assume this is due to the EGR/AIR delete and may go away with the plug in tricker thingy)
311 - Thermactor air system inoperative - right side (I assume this is due to the EGR/AIR delete and may go away with the plug in tricker thingy)
327 - Same as above

Cylinder Balance Test:
CEL flashed 9 times (believe this is a pass)
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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snrusnak

Member
Oct 11, 2020
7
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33
Gulfport, MS
Yeah like I said I've been through all that. I'm gonna try the ECT sensor, even though I don't have any codes thrown for it. I was talking with a buddy that knows a lot about the foxbody's and 5.0s and I didn't realize there is an ECT for the computer then a separate temperature sending unit for the gauge. I ruled out the ECT since the gauge appears to be working properly not realizing there were 2 sensors. Hopefully this will solve it, I don't really know anything else it could be...
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
One sensor, one sender, when doing 'the list' it (I believe) shows you how to test the sensor.
Some people jump around when doing the checklist, you do it step by step, the next step depends on the results from the one before.
 

snrusnak

Member
Oct 11, 2020
7
1
13
33
Gulfport, MS
Yeah I've basically gone through that checklist (and a nearly identical one from another site) about 3 times now. I have not actually tested the sensors though, which I will do if this ECT sensor doesn't work I suppose. I'm also not really able to find anyone who has the same issue (runs rough at cold start, then runs great when warm). Most are the other way around, or have a idle problem whether cold or hot.

Also, I'm new to this car, but from what I see there's an ECT sensor for the tuning/computer, and then a single wire analog temp sending unit for the gauge on the dash.
 

ameration

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Apr 8, 2002
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KOEO Codes:
327 - DPFE or EVP circuit below minimum voltage of 0.2 volts (I assume this is due to the EGR/AIR delete and may go away with the plug in tricker thingy)
I have the EGR plug if you’re interested. I just got a full tune so I no longer need the plug and I’m selling it. Please send me a private message if you’re interested.
 
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snrusnak

Member
Oct 11, 2020
7
1
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33
Gulfport, MS
So I've gone back through the whole checklist again, not sure if it's resolved or not yet. Here's what I've done:

Originally I cleaned and tested the TPS, IAT, ECT, and IAC. I believe that's every sensor except the MAF. Also cleaned TB and MAF. Pulled codes, reset idle, checked base timing, etc. All sensors checked good on their resistance and voltage per information listed (temp sensors checked cold and hot). Even tried a spare known good ECT from a buddy. The charging issue that the car originally had reoccurred and it dropped to only charging lke 11.75 volts, so we warrantied the duralast alternator as I am thinking the voltage regulator was faulty. Installed new alternator and that issue seems to be resolved. At the same time that I did the alternator I did the following:

- Cleaned and checked gap on all plugs. They are the correct plugs (autolite 25s I believe). Gap was a little tight compared to oem spec at like .040, which I doubt is the problem, but I opened them up to more like .054 as that's what I found for oem spec.
- Checked and rerouted all spark plug wires per a diagram someone posted that was from a f series/bronco, but it was to avoid crossfire due to wires running next to each other.
- Pulled cap and rotor. The contacts on the cap had a decent amount of corrosion, so I cleaned all the contacts up with scotch pad.
- The IAC looked brand new (previous person working on it probably changed it), but I had access to try another brand new IAC so installed it.

After this, I checked the MAF and everything seemed fine, but the car still seemed to be idling rough when cold. I didn't drive it so dunno about the bucking and running rough issue. As I finished doing checks on the MAF and was unplugging and plugging in the harness, I restarted the car partially warmed up and it seemed to run great. I let it cool down for a few hours to basically fully cold and started it up and drove it aroudn the block and it ran great. I'll test it again tomorrow morning to see how it does.

Problem is, if it is "fixed", I don't know what fixed it. Was it a poor connection at the MAF harness? Was it the corrosion in the distributor cap? Was it the voltage issue from the faulty alternator/voltage regulator? Spark plugs/gap issue? Spark plug wire routing/crossfiring issue? The previous IAC was faulty even though new? I guess I'll have to run it through some startup cycles and see how it behaves...

Any thoughts are appreciated.
 

snrusnak

Member
Oct 11, 2020
7
1
13
33
Gulfport, MS
ameration, this is going to sound ridiculous but I can't figure out how to send you a PM lol. I'm interested, let me know how much you want for it. Thansk
 

snrusnak

Member
Oct 11, 2020
7
1
13
33
Gulfport, MS
As far as computer control, what all does the ecm look at at cold start/warmup? I'm thinking ECT, IAT, MAF, and TPS. I beleive O2 sensors are ignored until at operating temp, so have not messed with them at all.

The bucking/misfiring only occurs when trying to drive it at a cold start and as it's warming up. It has a slightly rough but not too terrible idle at the same time. Once up to operating temp, it idles like a purring kitten and runs like a new car. So I'm thinking it's something to do with open loop base tuning. I did pull the computer today hoping I'd find a JET chip or some crap but there was no chip. Someone had been in there before though, as a clip on the door sill plastic was broken and one of the 3 harness connectors in font of the computer had it's retaining clip broken also. Maybe it previously had a chip. It's not possible to have a tune on the ecm without a chip, is that correct? In other words, if I don't see a chip, I can assume it's factory tune? Sorry for somewhat dumb questions, I've never really messed with OBD1 before...