Problems after exhaust install and 02 harness

djj62478 · Jul 4, 2026

Would an exhaust leak from the headers to the h pipe cause this issue. It's the only thing I can think of. I can feel by the ball and socket that one side of the flange is tighter than the other. So far I've checked for vacuum leaks found nothing but I don't have a smoke machine(never touched any lines when changing the h pipe and o2 harness though), checked timing with no luck, tried unplugging the mass air, O2 harness and iac no change. It's starts right up, surges up and down till it gets extreme then stalls, strong fuel smell and has trouble starting again without the help of the accelerator.

Praetorious · Jul 4, 2026

Sorry you haven't been able to get it squared out,if i lived near you i would offer to help but im several states away, no, a exhaust leak wouldn't do this, it would sound funny and yes,the car wouldn't run totally perfect but it wouldn't do what you mention,cars run fine with exhaust leaks all the time, have you unplugged your vacuum line to your regulator and see if you can smell fuel? i know you said you checked vacuum lines but have you checked under the intake with a flashlight? ,sometimes lines there get brittle and crack.

Kinda sounds like a fuel issue,be nice if you could check and see how much fuel you are getting at the rails.

djj62478 · Jul 5, 2026

Can anyone tell me what these connectors go to? The first pics are of a plug with a 2 wires green with black and black with yellow, it's near the coil area. The second plus is also 2 wires grey with yellow and red and it's near the passenger side fuel rail attached to another 2 wire plug. Just wondering if one of these not plugged in might be a culprit or not.

Noobz347 · Jul 5, 2026

What year is the car?

djj62478 · Jul 5, 2026

djj62478 · Jul 5, 2026

Also something new just occurred now the car will surge like crazy but if I keep feathering the gas to keep it alive after about 2 minutes it will stable out and idle at about 750 with no problems whatsoever then after about a minute of idling smooth with no issues it'll start to hunt again until it dies. Don't know if that helps anyone rule anything out? Oh and it's dumping so much fuel out will make your eyes melt....

AeroCoupe · Jul 5, 2026

Have you cleaned the mass air meter element?

djj62478 · Jul 5, 2026

AeroCoupe · Jul 5, 2026

Might want to check it. I use this and a q-tip:

CRC QD Electronic Cleaner

CRC QD Electronic Cleaner is a petroleum distillate / alcohol based precision cleaner designed as an alternative to CFC based cleaners. Plastic safe.

www.crcindustries.com

I believe there are videos on YouTube on how to do it. The element is delicate.

Praetorious · Jul 5, 2026

Those are the plugs for your charcoal canister (emisions), i believe they should be plugged together , unless the system is not there anymore, on my 91 last year i put all that stuff back on,it had been off for decades and i just got tired of the smell of gas in my garage and while driving.

Reading your comment a bit more carefully I'm certain picture number 3 is for the charcoal canister because that connector and the one for the temperature sensor for the PCM are wired very close to each other and it plugs in to a plug identical to the other pictures you show.

Praetorious · Jul 5, 2026

The other connector is for the air conditioning compressor, i assume its not working?

djj62478 · Jul 5, 2026

The car has no smog or ac.

Praetorious · Jul 5, 2026

Yeah, thats what those connectors are for,no worries there, thats not your problem, that was a great idea Aero suggested, clean that mass air meter, its a simple procedure, i can take mine off without taking the mass air system of the car,just unplug it,take your screws off, i believe they are torx and VERY GENTLY pull the unit up and out of the housing and spray a Qtip and gently clean the sensors and put it back together, those sensor can get pretty dirty.

Sometimes when we do a repair on our cars something else just breaks and its pure coincidence that happens to something totally unrelated to what we were working on originally, been there, done that,it happens, not saying this is the cause but have you ever had your computer out and looked at your capacitors? these cars are getting up in age where the computers are failing,most of the time will be a fuel pump that will stay running constantly or idle issues etc

AeroCoupe · Jul 6, 2026

Lots of threads on here with failed ECU’s just due to age. Most send them to the ECU Exchange.

1986-2004 Ford Mustang A9L A9P ECU Repair Service

1986-2004 Ford Mustang A9L A9P ECU Repair Service Engine Computer Repair Please fill out the R&R form here once you've placed your order.ORInclude a note in the box with your order number and vehicle symptoms.A Trusted Product with LIFETIME WARRANTY QUICK TURN AROUND TIMEDomestic and...

www.theecuexchange.com

djj62478 · Jul 10, 2026

Haven't had much time to mess with the car until yesterday so here's and update. I bought mass air cleaner, cleaned it and it started up, revved up to about 1300 quickly then dropped down and seemed to hold an idle. As I let it warm up it slowly crept up and down but very minimum. Then it started going from about 1200 to 500 or so and surged in that range, never dropped low enough to stall this time though. Went and pulled the mass air plug out feel on it's face and died. Started it back up pulled the plug and it ran as stated earlier. Cleaned the mass air again and sand situation. This time though when i pulled the mass air out started surging bad and died then loaded up on fuel and won't start again without help from the throttle. No vacuum leaks and I'm not sure if a bad O2 would do this.

Noobz347 · Jul 10, 2026

Well... It seems to me that you've eliminated the question of whether the MAF is feeding the ECU. - Check No reason to repeat that step.

It does not tell you whether the MAF is sending out good signals or not.

My next step would be to clear the CAM, hook it all back up and start doing ECU tests and pulling codes.

Do you have one of these?

https://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW/ref=sr_1_7?crid=1IZLV90B0HEME&dib=eyJ2IjoiMSJ9.3UdRDMaRpu8lFnF_b0i2Yl1NRkxf-10jtBp8bkEAYMClxSU9wHCaT_w0dKDJHF5wXueIbmCwNecVLR0XTtJSOt0n4JCKS3-iFwZhj-zPYuziVDsF5oT7NqjQj3nRbhLfiyvFnHXTr0ePQAoNGhbYvd1esQSDCUaR9ipJdqvrg5IGCY67uTq-PWWSXfOiTKDzq39u8icajeEFrb_nezz45MT8KCrY7OosokooBU6PKuVxgfUM7F6rGhLo15NNaENPQOKyc31uGgModmeQrzx-_4mlnB3w-Nk77_g9k_-4Wzs.LP0zqAB_-hjb3TIhweHDfXXQDVPMVZZQ7z7KXOgQj0w&dib_tag=se&keywords=obd1%2Bscanner&qid=1783708343&sprefix=OBD1%2Caps%2C184&sr=8-7&th=1

djj62478 · 2026-07-18T12:17:40-0500

Just got time to pull the codes. Key on engine off I got 85,51,66,96. Engine on I got 41,33,21.

General karthief · 2026-07-18T21:21:24-0500

Check the vac lines to and from the purge canister, check valve operation, I'm guessing a vacuum leak.

Noobz347 · 2026-07-18T22:02:27-0500

The first thing I notice is that the car doesn't seem to have been fully warm when you ran the tests.

After that, I would [highly] recommend that you run JRichker's Surging Idle Checklist:

Thread 'Help me create the "Surging Idle Checklist"'

Apr 10, 2005

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help.

Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of a systematic process that checks and eliminates possible causes. This idle problem checklist will use the same troubleshooting strategy and methodology.

By the term surging idle, I mean an idle that...

AeroCoupe · 2026-07-18T22:37:29-0500

KOEO
Code 51 - ECT sensor out of range indicated/circuit open
You can unplug the sensor and use a jumper wire across the plug. Run the KOEO again you should get a Code 61 of the wiring is good which eliminates the open circuit.

Code 66 - Mass Air Flow (MAF) sensor voltage is below the minimum allowable limit

Thread 'Code 66 Help!!!'

Oct 6, 2017

All,

I have a 90 Gt that I have completed a top end rebuild on. All new gaskets and hardware. When I got the car done it had a hell of a surging idle. Cleaning of the 10 pin connectors finally tamed that down to an acceptable level. The car has always had a code 66 for MAF voltage low since I have had it.

Here is the issue. The wiring for the MAF is good and shows no resistance to the computer. The signals all check out based on the surging idle checklist numbers. The power and ground are good. This is a new MAF stock replacement. The one on the car when I got It was also a stock unit. I...

Code 85 - Canister Purge (CANP) circuit failure

I have heard that you can use a 75 Ω 3-Watt resistor in the CANP connector and it'll clear the code 85. If that does clear the code then you know the solenoid is bad.

Code 96 - Fuel pump monitor circuit shows no power

Thread 'Anyone know how to fix a code 96? (Has to do with fuel pump)'

Apr 21, 2005

My buddy did a burnout last night at the top of my street in his stock 91 vert auto and then about a mile down the road his car died, LOL, but yea we need to fix it now. I pulled the codes with KOEO and got a code 96 which was fuel pump secondary circuit failure/high speed fuel pump relay open or something like that. We dropped the tank and replaced the pump with my old one, no luck. We then took out his driver seat and replaced the relay, no luck. We check the inertia switch in the trunk and it was fine. So if anyone has had this before and any luck in fixing it please lend a hand...

KOER
Code 21 - Engine Coolant Temperature (ECT) sensor is out of its self-test range

Same as above

Code 33 - Insufficient EGR flow detected.

Post in thread 'Codes 33, 87 & 96 - 89 LX?'

Feb 13, 2011

Code 33 - Insufficient EGR flow detected.
Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10” of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal flickers. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the...

Code 41 - O2 indicates passenger side system lean

Post in thread 'KOER Code = 41?'

Aug 19, 2005

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated...

Problems after exhaust install and 02 harness

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