Low Rpm Lean Pop Trouble

[mikestang63]

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.

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

 

[SloNoMoRacing]

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.

 

[A5literMan]

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

 

[SloNoMoRacing]

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

It better not be spun. It's a brand new SFI Approved 50oz unit from Romac (DSS Racing). I've tried multiple working MAF's, and even ran it without the MAF to troubleshoot. It had the same exact popping each time.

Pro-M told me it sounded ignition based too, but after going through the whole system it's now sounding more like a fuel issue. The only thing I still haven't done with the ignition is check the resistance of the plug wires. I may just yank the injectors and test them all individually. That will give me an excuse to send TMoss the lower intake for port work anyway.

 

[jrichker]

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.

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.

 

[KENS89LX]

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

 

[SloNoMoRacing]

I really wish it were a vacuum leak. I've sprayed everything from carb cleaner to straight ether all over the engine bay, and the idle wont change. All the vacuum lines are new rubberized tubing. The only place I could possibly see there being a vacuum leak would be somewhere in the lower intake gaskets. I've read where GT40X heads can have difficulty lining up properly with the gaskets at the lower intake. I've shot plenty of carb cleaner down in that area while idling with no change in idle speed though. So unless there's an issue between ports on the lower, I doubt it's vacuum. I'll find out for sure when I yank the lower off and send it for port work.

Thanks

 

[elarm1]

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?

 

[jrichker]

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

 

[mikestang63]

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.

 

[SloNoMoRacing]

Well... Before I respond to the last couple posts (cause I think you guys are dead on with part of the problem), let me update with what happened tonight.

Since recently it's been running pretty well, I drove it a couple hours to Palm Beach International Raceway, to get a baseline 1/4 mile MPH trap speed and basically play with it a little before doing any port work or other tweaking. I ran consistent 104's btw, and trapped at 13.05 best on street tires. On the way home I stopped for gas, and noticed my fuel pump audibly howling loud as hell. I shut it down, and when I went to start it again I had nothing. My brand new Walbro 255 in tank crapped out. So I started checking wires, and when I got to the fuel pump relay it was hot enough to burn my hand. I tapped it a few times and got the pump to turn on again. I was actually able to limp it home (120 miles), and it's DOA in my driveway now.

Guess my first order of business is to chase some wires tomorrow, but it's looking like this whole popping gremlin may be due to either a crappy connection, or a dead pump. Hopefully this irons out this issue!

About the vacuum leaks... I just went through my build list yesterday, and realized that I used felpro 1250's instead of the 1262's. I know that the GT40X's require the 1262's, and I was pretty sure that's what I used. Apparently not though. I've seen the posts about this exact mistake too, but I guess the lower needs to come off now so I can put the right (large port) gaskets on and not have to worry about it.

Thanks again everyone. I'll update after I iron out the fuel pump issue, and swap out the lower intake gaskets for the right size ones.

 

[Zbanks1]

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

 

[SloNoMoRacing]

Okay so I'm back to close out this thread with the final update. Turns out it was a culmination of multiple issues, and they've all been addressed. The fuel pump wiring was only part of the problem. As it turns out, the lower intake gaskets weren't doing their job. Coolant was migrating into the intake runners, and so was oil. Coolant was also seeping out and running down the back of the block. It wasn't visible since it was burning off before it could puddle anywhere. I could smell it though, and I was obviously concerned that I had a head gasket blown. I'm guessing the coolant was causing spark blowout, leading to popping from fuel loading up in the cylinder.

Btw the gaskets were in fact the smaller felpros, and not the large port gaskets required for the gt40x heads. Whoops. I have the right ones now, so I can finally put this to rest and move on.

I also noticed the my Pcv screen was missing/not installed, and oil was creeping up into the intake.

Thank you for all of your input. Lmk if anyone has any questions, but this issue is resolved!