Low Rpm Lean Pop Trouble

SloNoMoRacing said:

Distributor didnt do it either... Guess it's time to focus on the fuel system. I have an A9P computer that I'm going to try as well just in case. I replaced my original A9L when it fried with an 8LD, and i just want to be sure its not hiccupping.

What sucks is that this may boil down to a simple wiring issue. I've been out of the foxbody realm for awhile now (been driving an SN95), and I'm now remembering all the wiring gremlins these cars can have.

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Hey if you get stuck and have run out of options. Give the guys at GAW in Miami a call. Gabe knows his stuff and is a great guy.

http://www.gaw4x4.com/gaw-racing.html

Is the balancer spun? Throwing off your timing? After all your tests it sounds like ignition/timing or maf based. Weird

SloNoMoRacing said:

Grabbin Asphalt: Yep, tried all those. The fuel pressure is steady when's it's popping like that. I've tried it with vacuum on and off too, with the same results.

Mikestang63: GAW as in Gabby's Auto Works? Wow I didn't know they were still around! That was a popular place for Mustang guys around here when I had my first one back in the 90's. I thought they got into 4x4 stuff after that though. Thanks for the heads up.

So just playing around with it today I decided to increase the timing to 18 degrees initial again, and the backfiring/popping almost completely cleared up... It ran incredible too. So then I turned it up to 20 degrees and it finally didn't like it, but it still didn't detonate. What the crap does that mean? Am I just way over injected with the 24 pounders? Like I said before, I've never seen a foxbody take more than 14 degrees of timing without detonating like crazy and running like crap. Why would this build run better with so much more initial timing?

One thing I'm not sure if I mentioned before or not is that when decelerating in gear, it also backfires like a madman. Loud, and flamey backfires. Like the cylinders are loading up with unburnt fuel.

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Interesting point. The TPS circuit is supposed to tell the computer to decrease fuel flow when decererating.
HIgh RPM compared to throttle position would tell the computer that the engine is in decel mode and trim back on the fuel delivery.


TPS Troubleshooting
Setting the TPS at .98 or .99 volt is a Bozo Internet Myth. When the computer powers up and initializes the TPS sensor, whatever it reads is the zero starting point for it. In other words your .98 volt becomes 0 volts and the computer watches for increases in voltage from that point upward.

Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy. Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.

Tools needed:
Analog voltmeter or a digital voltmeter with an analog bar graph. A digital voltmeter will average out a varying voltage reading.
Small screwdriver
Solvent for cleaning wiring connector pins


Wire colors & functions:
Orange/white = 5 volt VREF from the computer
Dark Green/lt green = TPS output to computer
Black/white = Signal ground from computer

TPS troubleshooting steps:
1.) Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.
2.) Use the Dark Green/lt green & Black/white wires to set the TPS base voltage. Try this... All you need is less than 1.0 volt at idle and more than 4.25 at Wide Open Throttle (WOT). You'll need a voltmeter with a 1 or 3 volt low scale to do the job.

The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running. Set the voltmeter on the 2 volt range if it doesn’t auto range.

Here’s a TPS tip I got from NoGo50

When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

(copied from MustangMax, Glendale AZ)

A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.

TPS will not go below 1 volt
Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 1.5 ohms. Higher resistance than 1.5 ohms indicates a problem with the 10 pin connector, computer or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.



See the graphic for the location of the 10 pin connectors:
Diagram courtesy of Tmoss & Stang&2birds



See the graphic for the 10 pin connector circuit layout.

Unplug the white 10 pin connector to do some resistance testing. It is good time to clean the connector pins and examine the connector for corrosion, broken wire or other damage. See http://oldfuelinjection.com/?p=85 for help in this department.

If the resistance on the TPS Black/White wire and pin 1 of the white engine fuel injector harness 10 pin connector, is more than 1.0 ohm, you have bad connection or broken wiring. Repeat the test using the pin 1 of the white body side 10 pin connector and battery ground. You should have less that 1.5 ohm. More than that is a damaged signal ground inside the computer or bad connections or wiring.

Vacuum leaks will do that. had similar issue, found a leak in one of the lines near firewall.

My money is on that C&L. Ever since i put that thing on my car I got the lean "pop" when goosing the gas sometimes. I just learned to live with it until my set up changes. It has me curious as i have those same issues... Do you have a video of it that you can post?

Not to compound your misery, but...

Vacuum leak due to slipped lower intake manifold gasket...


Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.
http://www.gotstang.com/photodisplay.php?iid=111113

Take the intake tube off the TB and with the car running, put your hand or a rag over the TB opening. If the car keeps running, chances are you have a vacuum leak.

Did changing the o2 sensors help the lean popping issue? My car is doing the same thing.