93 Mustang EFI acceleration stumble

[stretch]

Stabbed a Msd pro billet dissy with a known good tfi. Actually ran worse.

 

[MD]

I would add an additional ground between the engine ground for the ECM (there is only one ground on the RJM harness) to a new body ground location. It should connect directly to the same stud or bolt on the engine that the ring terminal ground on the RJM harness uses. It may not help, but it is an easy way to alleviate the most likely ground issue.

The second thing I would do is check the continuity (resistance) between the metal case of the O2 sensors and the RJM harness ground, just to make sure they are 0 Ohms. The A9L is a 3-wire. One for the 02 sensor signal voltage, and two for the heater. Sensor ground reference is the exhaust itself. The factory harness has a separate 02 Sensor ground from the computer to the exhaust, but the RJM likely ties it together with the single ground. Also disconnecting the O2 sensors would be an easy test also.

The engine doesn't need to warm up to go into closed loop. Heated O2 sensors can start performing after only 60-90 seconds. I think the real issue that you are likely facing is that when the ECT sensor reaches a certain level, the adaptive functions will be enabled. On the A9L, adaptives are less than ideal and can cause some real headaches at idle. If you can datalog the sensors, I would look at the O2 sensors, the MAF voltage, and the TPS. Even a 10% variance can cause the adaptives to really jump the fuel around and cause stumbling idle. If there are issues, I prefer just to disable adaptive below 1500. If the TPS voltage is off slightly, and the ECM thinks it is above idle that can also make adaptive worse.

 

[stretch]

Good information here, looks like Ryan’s old site.

https://www.grandmarq.net/oldfuelinjection/page1.html

 

[DirtDonk]

What would be the symptoms of a bad BAP sensor? When they go bad I thought it just wouldn’t run at all. But maybe there are variations on that theme?

 

[67EB_in_619]

Can you pull all the sensors other than TFI, let it run on tables, and then plug in the sensors one at a time and see which is giving a false reading causing the stumbles once warm? I would start with MAF and O2 (pretty sure there is a fuse for them on the RJM harness?)

What is your voltage at the coil and have you observed spark strength??

 

[Broncobowsher]

I'll throw out there to not forget the basics. Compression test, in this case hot and cold. Leak down?

 

[stretch]

Well verified the grounds. I already had a second ground on top of the RJM harness ground. Ground to body then to frame.

I will give the sensors idea shot tomorrow. I just remembered I had a similar problem when I finally had the Bronco back together enough to start. It was a bad MSD coil, swapped them out with another MSD coil and all was fine. I believe I have a Ford coil I can swap.

I hope I don’t have a problem with compression. The motor only had 32k on it when installed and compression as solid all cylinders.

 

[stretch]

Replaced the coil, no change. Running engine, closed loop, pulled each sensor individually, and no change. I pulled the fuse for the O2’s and no change.

The only item at this point that I haven’t checked is the fuel system. Maybe I have a semi clogged filter? I’m running a GM style in tank pump, so I’m assuming that hoses etc… aren’t breaking down inside the tank Causing the problem.

I am picking up another computer tomorrow. This one has been rebuilt and is known to be good. It could be possible I have two defective ecu‘s as neither has been rebuilt.

 

[stretch]

Replaced the fuel filter, no change. Looked at number 7 plug wire, it’s a little burnt on the out side but doesn’t appear to go through. Has a sock on it as well

I guess ill grab the new computer and hopefully it solves my problems.

 

[73azbronco]

I do have a pressure gauge on the fuel rail. 30 at idle, 40ish at cruise. Checked the grounds, these are all new, engine to frame, frame to body, front and back and body to engine. Seems fine.

So during start and warm up, idles fantastic. Once it goes closed loop, falls flat on its face. Idle starts to hunt and runs rough.

35 to 40 is what I'm finding for a 93 stang setup.

 

[Broncobowsher]

Refresher in injector pressure and the regulator.
The injectors are rated at 42.5PSI delta. That is pressure change between the feed and the tip.
With manifold vacuum there is a greater pressure delta across the injector. 20" of vacuum is about 10PSI of pressure. So at idle and other high manifold vacuum conditions you are getting 52.5 PSI across the injector with a fixed pressure regulator. The regulator being vacuum referenced will reduce the fuel pressure by the same amount as there is manifold vacuum.
So the gauge reads 32.5 PSI at idle with 20" of vacuum. What the injector is flowing is still the 42.5PSI between the fuel rail and the intake manifold.
The computers that ran this setup were not that powerful. To put in the computing needed to run a fixed pressure injector and let the ECM make up for the different pressures/flows through the injectors took a long time to come. When the non-return fuel system came out, they finally had enough computing power to deal with it.

The unplug the regulator as a test makes for a higher flow rate out of the injector at idle. An artificial rich condition. Doesn't change anything under load.

 

[stretch]

If you are asking if the is any change, no there isn’t. Vacuum remove from the regulator doesn’t change the stumble, maybe changes the rpm slightly. 32 psi to 40 unplugged, which is were I’ve historically been running before the problems began.

Answering a previous question, there isn’t a smell of gas on the vacuum line for the regulator.

 

[EFI Guy]

Your 40 psi is fine. If we're splitting hairs the actual spec is 39.15 psi delta.

Don't discount the plug wire with a burnt end. I just replaced one that didn't look that bad yesterday but it solved the misfire under load.

 

[blade]

I agree with Efi Guy, I would test all of the spark plug wires also not just visual inspection.

 

[stretch]

Will do. I did install a known recently rebuilt A9L. No change. I’ll take a look at plug wires again.

 

[stretch]

Replaced number 7 spark plug wire. Problem solved. I need to find a better solution for this one plug.

Thanks to everyone that had suggestions. This was a lot of extra work, but it was a good refresher for me on an older efi system.

 

[DirtDonk]

Great news! Thanks for the follow up.
Shows you how just a little bit of electricity not getting through, or going in the wrong places, can make a big difference.

 

[DirtDonk]

In fact, this just proves that the old saying applies just as much to fuel injection as it does to carburetors.
To paraphrase: “90% of fuel delivery problems, are electrical!“
The more things changed…