My mustang is bucking while driving

MARKSSTANG · Jul 25, 2007

I recently put in a forged long block from MMR, with a compression ratio of almost 12:1. I got a XCal tuner to help adjust with the timing, and I still have to run octane boosters on top of 91 to keep it from knocking. Now, I am getting a check engine code P0452 which is "evaporation system fuel tank pressure sensor circuit - low output." My car is also acting funny while driving. It's an automatic and while driving at a constant speed (around 40 is most noticeable) and a constant throttle position, the car bucks back and forth, like it can't figure out whether it should shift or not. Is there something wrong with my tune? Is my engine misfiring? I've replaced my gascap, the fuel pump, and the sensor circuit to try to fix that code, and it still comes back. Any advice would be greatly appreciated!!!

Mark

LI98GTStang · Jul 25, 2007

Tough one you got there. Did you replace the fuel filter when you did the pump? Check for leakage/cracks around the hose that connects to the gas tank filler? Maybe retard the timing. I know on my old Honda if the timing is too far advanced, it can "buck". Good luck with the diagnosis.

MARKSSTANG · Jul 26, 2007

Thanks for the reply. I also wanted to add that the car feels ok when you let off the gas, or while you're accelerating. It's a weird problem, but I can't seem to figure it out!

And with reguard to timing, I have had it taken down a lot to try to run on regular pump gas (without octane boost). Now that I'm running a powerful boost and 91, could it be that the timing is too retarded? Should I advance it further?

fast97gt · Jul 26, 2007

are you running the proper size injectors? and also you could have an o2 sensor acting up, and the computor maybe cant detect it cause it doesnt do it constantly at all rpms. and it also ocured to me that it could be the egr valve if you still have one.

DerekStangGT · Jul 26, 2007

MARKSSTANG said:
I recently put in a forged long block from MMR, with a compression ratio of almost 12:1. I got a XCal tuner to help adjust with the timing, and I still have to run octane boosters on top of 91 to keep it from knocking. Now, I am getting a check engine code P0452 which is "evaporation system fuel tank pressure sensor circuit - low output." My car is also acting funny while driving. It's an automatic and while driving at a constant speed (around 40 is most noticeable) and a constant throttle position, the car bucks back and forth, like it can't figure out whether it should shift or not. Is there something wrong with my tune? Is my engine misfiring? I've replaced my gascap, the fuel pump, and the sensor circuit to try to fix that code, and it still comes back. Any advice would be greatly appreciated!!!

Mark

i would start with the obvious and go from there. first replace your fuel tank pressure sensor. im not sure on fords (maybe a ford tech can chime in) but on Hondas you usually have to replace the bypass solenoid, 2 way check valve, and canister vent shut valve. after that, if its still bucking, try to get some freeze data and look at your EGR valve. normal should be around 2.05 volts. it may be stuck open or closed. but my guess is the fuel tank pressure sensor is causing a problem with your emmisions systems, causing the bucking

bakos24 · Jul 26, 2007

I would check your o2 sensors. My car did the same and it turned out to be a bad o2.

Yellow2002GT · Jul 27, 2007

There is alot of variables to check at this point...are your heads ported and are you running a cam.........sounds like timing is the key point here........without race fuel in a 12.1 compression motor you are going to have to bump the timing way up! I would delete the EGR and go one step colder on your plugs......pull a plug and see if it has burn spots on it.....I think the timing down low is effecting it!

"ED"

DerekStangGT · Jul 27, 2007

no, start with the obvious. you have a code for a fuel tank pressure switch. replace that

MARKSSTANG · Jul 27, 2007

Sorry for the delayed reply. I have already replaced the sensor circuit, fuel pump, and gas cap and the code still comes back. I am wondering if the code is even related to the problem? I appreciate the advice thus far, and I will get back to you with recent updates.

Mike97gt · Jul 28, 2007

MARKSSTANG said:
I recently put in a forged long block from MMR, with a compression ratio of almost 12:1. I got a XCal tuner to help adjust with the timing, and I still have to run octane boosters on top of 91 to keep it from knocking. Now, I am getting a check engine code P0452 which is "evaporation system fuel tank pressure sensor circuit - low output." My car is also acting funny while driving. It's an automatic and while driving at a constant speed (around 40 is most noticeable) and a constant throttle position, the car bucks back and forth, like it can't figure out whether it should shift or not. Is there something wrong with my tune? Is my engine misfiring? I've replaced my gascap, the fuel pump, and the sensor circuit to try to fix that code, and it still comes back. Any advice would be greatly appreciated!!!

Mark

First thing you need to get back to basics... That code is a evap system code and has nothing to do with your bucking.

First thing to check on ANY modular regardless of mods when your bucking is your plugs and coils. I have seen cars come in with brand new non motorcraft plugs and have misfires that were plug related. Also check your coil boots and check for water in the plug wells..

Quickest way to get a misfire to happen is drive up a hill at around 35-40 with the converter locked. If it starts bucking then it's a ignition related misfire. Low RPM's and high loads is when you will notice a misfire the most. High rpms or low load situation you will not notice it as much.. Unless it's a type A misfire then it's a dead miss and you will throw a code for what cylinder is misfiring.

After you get your misfire figured out I can try to guide you through the pinpoint tests for the code you have.

MARKSSTANG · Jul 28, 2007

I have an appointment to take it in this week to have it checked out, but I have another little tidbit to throw out to your minds. When I'm stopped at a light and it's idle-ing the rpms bounce between 500 and 700 (between the .5 and 1 lines). It doesn't hold a constant rpm

ximportdriver · Jul 29, 2007

MARKSSTANG said:
I have an appointment to take it in this week to have it checked out, but I have another little tidbit to throw out to your minds. When I'm stopped at a light and it's idle-ing the rpms bounce between 500 and 700 (between the .5 and 1 lines). It doesn't hold a constant rpm

check the MAF and IAC for cleaning

Az Pete · Jul 29, 2007

Just a shot in the dark but my "00 Vert was doing the same action and I found that the A/C system was low on freon and it was the compressor kicking on and off. Added freon and it smoothed out. Never had a code from it.

Mike97gt · Jul 30, 2007

Let us know what you find..

nyuk98gt · Jul 30, 2007

MARKSSTANG:

EGR can cause surging if it is set too high. Dashpot functions can do the same thing, too. It could also be the torque converter locking/unlocking. Test this by stepping on the brake a little (just enough to put the brake lights on) and if the bucking/surging stops, it is because the t/c is unlocked (when brakes are applied). Have you reset the computer? Have you datalogged (MAF counts, SPARK, RPM, STFT's, LOAD, ECT (coolant temperature), IAC integrator/correction -- these are the things to select in the XCal for logging)? The datalog should be pretty informative.

You don't say whether you have cams but getting the idle stable with big cams requires some tweaking and you either need a tuner who knows what to do or you can fiddle around with SCT's Advantage (Pro Racer software) and get the idle tamed. There are quite a few things that affect idle!

/// edit: OK, re-read your original post. Get the evap system problem taken care of first. There's a little flow meter (red and white, iirc) and it is common that it conks out. Not expensive to replace, either. The evap system may be allowing more air in that needed causing an unstable idle. I would not expect it to be a problem at 40 mph, though.
end of edit ///

I hope the problem is just the t/c lock/unlock schedule -- it's easy to fix.

Chris

fast97gt · Jul 31, 2007

also check for large vacuum leaks between the maf and the intake.(t.b. gasket, upper plenum to intake gasket, holes or loose air ducked tubing etc) also check the dpfe sensor for carbon buildup in the lines ittl be near the egr valve.

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