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Old 05-10-2006, 09:22 PM   #1
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Engine sputtering and bucking at WOT

I recently had the following mods installed on my 331 : accufab 70mm throttle body, PMAS 80mm MAF meter, 30lb fuel injectors, and an AFM power pipe. The car idles much, much better now and it clearly has a lot more power. The only problem is that when you go to WOT in any gear, the engine starts bucking really bad and won't accelerate at all. Everything is fine under acceleration at part throttle up to 80-90% throttle, but once you go wide open, the thing goes crazy. Needless to say, this takes all the fun out of owning a fox mustang. Any ideas as to where to start looking to track this problem down? TPS maybe?
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Old 05-11-2006, 12:35 PM   #2
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Pull codes, for starters.

Verify tps voltage is linear and consistant throughout the movment of the throttle.

Verify there are no vacuum leaks

Verify/clean MAF sensing wires with CRC electrical contact cleaner.

Verify fuel pressure is at 39PSI/no vacuum. Verify fuel pressure stays up during WOT operation.

Honestly, sounds like a fuel issue. How big is the fuel pump, and how old is it?
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Old 05-11-2006, 02:25 PM   #3
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fuel pump is 255lph, about 2 months old. I also have an adjustable fuel pressure regulator that was just installed with the other mods. Perhaps it is not adjusted properly.
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Old 05-11-2006, 02:29 PM   #4
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Set the FPR at 38psi, vacuum off.
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Old 05-11-2006, 06:02 PM   #5
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It Sounds like your FP Is not right try playing with it go up a bit but no lower than 38lbs
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Old 05-11-2006, 06:27 PM   #6
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Changed the fuel filter lately?
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Old 05-12-2006, 08:29 AM   #7
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Quote:
Changed the fuel filter lately?
That was my next guess.
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Old 05-12-2006, 08:43 AM   #8
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Quote:
Originally Posted by MFE
Changed the fuel filter lately?
^^^^ X3......
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Old 05-12-2006, 09:20 AM   #9
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no, i haven't changed the fuel filter since i've owned the car (about 2 years, but less than 10,000 miles). I'm leaning towards the TPS though because the engine sputters at WOT regardless of the rpm. If i'm driving at 2500 rpm and mash the gas, it still starts bucking. Could a bad fuel filter cause this?
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Old 05-12-2006, 09:27 AM   #10
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Failing fuel system could cause it. Dirty/contaminated MAF sensing wires could also cause it...
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Old 05-12-2006, 03:20 PM   #11
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I'm gonna go buy a fuel filter and see what happens. The MAF is a brand new PMAS so hopefully it isn't dirty yet.
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Old 05-12-2006, 05:47 PM   #12
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Put in the new fuel filter..... same deal as before. TPS voltage looked ok at idle, but my volt meter kinda sucks. I have no idea what the TPS is doing at part throttle up to full throttle. My 255lph fuel pump, 30lb injectors, and now fuel filter are brand new. I'm still stumped. A Mustang that cannot be driven with the pedal to the floor is not one that I would like to know.
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Old 05-12-2006, 06:26 PM   #13
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Don't worry about spending on the fuel filter...it's maintenance anyway....a few of the alternatives to do are pending:

Quote:
Originally Posted by 93Cobra#2771
Pull codes, for starters.

Verify tps voltage is linear and consistant throughout the movment of the throttle.
http://www.corral.net/tech/maintenance/eecivtest.html
borrow a good VOM. LUK
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Old 05-13-2006, 08:06 PM   #14
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here are the KOEO codes:
11- all OK
continuous memory:
66, 96

the code 96 is probably from disconnecting the fuel pump relay when i replaced the fuel filter.
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Old 05-13-2006, 08:40 PM   #15
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Another code 66 that might be causing the problem....suggestion, do a clear codes, rerun KOEO self-test to confirm 66 and 96 have been cleared....do a couple of test drives (normal, stop-n-go and WOT runs), and run a KOEO self-test to check if code 66 shows up again. BTW, how did the TPS check go?...LUK
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NOW: 5.0L Stock shortblock, X303 Cam, SP Offy. Port-O-Sonic, 700 cfm DP Holley, 289 '66, 1.7 RR, Richmond Gear 4+1 Trans., Centerforce DF kit, 2.73 Gears

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Old 05-14-2006, 04:21 PM   #16
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continuous codes have been cleared for now and KOEO shows 11. KOER codes show 98 and 66. I have a feeling that the MAF must be causing this. Is it possible that it wasn't calibrated correctly from PMAS? I will try to clean it with some contact cleaner later, but it is brand new.
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Old 05-14-2006, 05:35 PM   #17
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Quote:
Originally Posted by perkstang
continuous codes have been cleared for now and KOEO shows 11. KOER codes show 98 and 66. I have a feeling that the MAF must be causing this. Is it possible that it wasn't calibrated correctly from PMAS? I will try to clean it with some contact cleaner later, but it is brand new.
Bad MAF calibration sounds as the culprit....KOER codes 98 & 66 could be considered confirmation codes....you might want to make sure it's not a wiring issue, by checking MAF output, VPWR, and ground. Since this is a MAF calibrated-for-EEC-IV situation, suggest you do the following tests (last voltage check engine idling):



LUK what results you get...
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NOW: 5.0L Stock shortblock, X303 Cam, SP Offy. Port-O-Sonic, 700 cfm DP Holley, 289 '66, 1.7 RR, Richmond Gear 4+1 Trans., Centerforce DF kit, 2.73 Gears

WIP: 3.90 Gears(Motive Performance), LS Overhaul (FRPP), Adj. LCA's (New Design Granatelli Aluminum), Adj. UCA's (Granatelli), Caster/Camber Plates (MM), Subframe Conn. (MM) T-Top weatherstriping and rear braces bodywork
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Old 05-15-2006, 04:16 AM   #18
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Let us know. I am dealing with an exact same problem that recently reared its ugly head, except I have speed density. Car is smooth as silk if I rev the car slowly to 6k. If I go WOT, car instantly falls on its face and bucks/ sputters. Highly embarassing. Like you, all my fuel system components are brand new front to back.
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Old 05-15-2006, 05:12 AM   #19
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If you haven't verified and adjusted your fuel pressure and timing correctly you are looking around for something that probably isn't there......T U N E is E V E R Y T H I N G!
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Old 05-15-2006, 01:06 PM   #20
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fuel pressure was around 35, then we bumped it up to around 42. No change. Timing is set to 14 degrees i believe.
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Old 05-15-2006, 02:56 PM   #21
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Quote:
Originally Posted by Coupester
Let us know. I am dealing with an exact same problem that recently reared its ugly head, except I have speed density. Car is smooth as silk if I rev the car slowly to 6k. If I go WOT, car instantly falls on its face and bucks/ sputters. Highly embarassing. Like you, all my fuel system components are brand new front to back.



http://www.onr.com/user/pbooth/mustang/tfi.html

Ohm your TFI
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Old 05-15-2006, 10:15 PM   #22
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Quote:
Originally Posted by perkstang
fuel pressure was around 35, then we bumped it up to around 42. No change. Timing is set to 14 degrees i believe.
You have a relatively healthy 331.....try up to 50-55psi before settling on anything with only 30's. Depending on cam and compression you may need up to 18-19 initial on the timing as well......these are THE first things you need to iron out before you go looking for culprits and a "problem".
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Old 05-16-2006, 12:05 AM   #23
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Quote:
Originally Posted by EvilGooses
GREAT page!!! Thank you for posting that!
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Old 05-16-2006, 12:19 AM   #24
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while i realize that it is definately possible to add more performance with added timing and fuel pressure. I don't think that this has anything to do with the problem i'm having right now. When i'm driving the car, I can put the throttle to 90% open and rev up to 6000 rpm. However, if i go wide open, the car will sputter and not accelerate. If i go to WOT, then let back to 90%, the car will sputter until i let off to 90% and then it will continue to accelerate normally. I am not an expert by any means, but this problem does not seem to me to be caused by a lack of timing or fuel pressure. I plan on taking the voltmeter and checking the MAF connection. When i took a look at the connection today, i noticed that the plastic piece that clipped the connector together was broken off, so it is possible that there was a bad connection. Also, the MAF was positioned upside down compared to stock (the MAF connector was at the bottom). I'm not sure if the way the MAF is "clocked" can affect the voltage output at all. Once again, I did not install these parts because I was out of town at the time.
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Old 05-16-2006, 12:52 AM   #25
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Check your TFI and distributor. That is what I plan on doing.

My symptoms are 100% the same as yours!!!!!!!!!!!!!!!!!!!! 90% throttle, it's fine!!!! 100% throttle it stutters and falls on its face!
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Old 05-16-2006, 08:36 PM   #26
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MAF wiring voltages look to be in spec. TPS voltage climbs relatively steadily with the opening of the throttle body, so I will rule that out for now. Next I might take out the MAF and have a look at it. I might also try the TFI check and see what i get.
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Old 05-16-2006, 09:24 PM   #27
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Quote:
Originally Posted by perkstang
When i'm driving the car, I can put the throttle to 90% open and rev up to 6000 rpm. However, if i go wide open, the car will sputter and not accelerate. If i go to WOT, then let back to 90%, the car will sputter until i let off to 90% and then it will continue to accelerate normally.
Key info there ^^^^....suggest you clear codes, confirm "clearance" by running a KOEO, and do some test drives to get the symptom to show up. Recheck if code 66 repeats....while you're doing the TFI check, you might want to check the condition of the vanes rotor in the dizzy, and see if there are signs of the rotor rubbing against the stator sensor, so the PIP and CID signals might not be having noise problems.....

TPS....I think you should monitor voltage output real-time....JIC. My
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1986 Mustang GT w/T-Tops

NOW: 5.0L Stock shortblock, X303 Cam, SP Offy. Port-O-Sonic, 700 cfm DP Holley, 289 '66, 1.7 RR, Richmond Gear 4+1 Trans., Centerforce DF kit, 2.73 Gears

WIP: 3.90 Gears(Motive Performance), LS Overhaul (FRPP), Adj. LCA's (New Design Granatelli Aluminum), Adj. UCA's (Granatelli), Caster/Camber Plates (MM), Subframe Conn. (MM) T-Top weatherstriping and rear braces bodywork

Last edited by Joel5.0; 05-16-2006 at 09:26 PM.
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Old 05-17-2006, 07:58 PM   #28
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I'm about to start taking out the TFI module, which looks like it's going to require me to take out the distributor. Anything else I should be checking while i'm at it?
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Old 05-17-2006, 08:35 PM   #29
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This might sound Silly, but it happened to me. I had the exact same problem. When I changed my MAF, My car drove fine,idled fine, everything checked out fine. But at WOT it bucked and sputtered like a mule.My problem was due to fact that the MAF plug wasn't seating good to the new MAF. It just wouldn't stay tite. Check it out, it might be as simple as that. I was so relieved. I put a zip tie on the pigtail real tite and then snapped in on. GOOD LUCK!!
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Old 05-18-2006, 12:08 AM   #30
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perkstang, just to give you a little update on my problem. I pulled codes this morning and found nothing. So, I went to the junkyard and pulled the distributor out of an 87 GT s**tbox. Turns out, it had a replacement TFI module already on it. I cleaned it up and popped it in my car. Voila, the problem went away. Car runs better than ever. I can't say whether or not it was my distributor or my TFI module, but both are replaced now. It was cheap too, only 20 bux for the whole deal.
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Old 05-18-2006, 12:10 AM   #31
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thanks for the input coupster.... i pulled the distributor, but have yet to check the resistance on the TFI itself. I'm gonna try to go to the mustang shop up the road tomorrow and see if i can get a replacement distributor to try out. You're giving me some hope... i'll let you know what happens tomorrow
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Old 05-18-2006, 06:57 PM   #32
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ok, i checked the resistance on the TFI and everything checks out as per the resistance values on the site posted above. Of course, that does not rule out any other distributor malfunction or anything else. The MAF connections look really solid. Is there any way I can test the distributor to see if it's functioning properly? Any other ideas?
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Old 05-18-2006, 07:20 PM   #33
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Did you also check the Hall Sending Unit?....(at the bottom of the link EvilGooses posted).
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Old 05-18-2006, 08:50 PM   #34
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I tryed to check the resistance between the "A" and "C" terminals on the Hall Sending unit but it wouldn't register anything on my meter. Does that mean the sending unit is definately fouled up? I am now going to try to test the voltage from the PIP signal.
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Old 05-18-2006, 09:08 PM   #35
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Ok, so I just checked the voltage between the MAF sig (blue wire) and the battery and got .07 volts. Is there a way to confirm a faulty MAF sensor?
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