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Old 06-11-2004, 12:10 AM   #1
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Stumbling during acceleration/ bucking at low rpm

Recently I noticed that my fuel pressure (under the hood gauge) was at 40 psi without vacuum. My cars does a little idle surging so I reduced the pressure to 38 PSI.
It seemed to help the idle. A day later I noticed alot more bucking then normal when attempting to cruise at around 1800 rpm. I need to keep rpms at around or above 2500 rpm or experience bucking.

I checked the fuel pressure and saw that it was now 34psi without vacuum. I turned it up to 38 psi, but with the same problems.

At light accelleration/ light load I feel a small amount of bucking/hesitation. When trying to cruise at 1800 rpm I experience the same thing. When accelerating hard I do not have the hesitation/ bucking.

I have a 190 lph pump with 3000-4000 miles on it and a T-rex with 1000 miles on it.

I also noticed that when I turn the key on I immediately have 32 psi with vacuum. When I turn the engine off the pressure drops to about 24 psi and seems to hold there. Should there be a pressure drop and how much of one should there be??

Any suggestions for my "new" problem?

btw....prior to this I would only have bucking at less then 1400 rpm when I would lug it down the driveway trying not to wake up my neighbors.
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Old 06-11-2004, 02:05 AM   #2
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vacume leak?

what is your vacume at idle?
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Old 06-11-2004, 02:38 AM   #3
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Quote:
Originally posted by BLiND
vacume leak?

what is your vacume at idle?
Vacuum at idle is 10. Which is where it has been ever since I installed the gauge, probably 1000 miles.

During the stumbling problem it is 15-18 psi.

Under a hard acceleration, 0 psi or when building boost it is not an issue.
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Old 06-11-2004, 10:53 AM   #4
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Have you made any changes at all to your car recently? Changed anything on your tune at all?

Almost sounds like your maf transfer curve got fouled up or something. Are you using 1.3 alpha with your tweecer?

You might want to throw this question up on the tweecer bbs as well...
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Old 06-11-2004, 11:32 AM   #5
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Have you checked\spread out the male pins in the "10 pin connectors" also known as the "salt & pepper shakers"? I just went through this & the symptoms sure do match what my car was doing......
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Old 06-11-2004, 11:33 AM   #6
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whoops..how did this happen? Double post-sorry.
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Old 06-11-2004, 01:20 PM   #7
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Quote:
Originally posted by 93Cobra#2771
Have you made any changes at all to your car recently? Changed anything on your tune at all?

Almost sounds like your maf transfer curve got fouled up or something. Are you using 1.3 alpha with your tweecer?

You might want to throw this question up on the tweecer bbs as well...
Absolutely no changes have been made to my tune.

I still have the old version.

I have not had a laptop for several months so there hasn't been any tweaks that may have occured by accident!

Last night I was driving home and the same thing was occuring. I took off the vacuum line to the AFPR and the problem ceased.

I can still hear the whine of the T-rex, but nonetheless it appears that I have sometype of fuel delivery problem.
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Old 06-11-2004, 01:21 PM   #8
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Quote:
Originally posted by AnthonyDalrymple
Have you checked\spread out the male pins in the "10 pin connectors" also known as the "salt & pepper shakers"? I just went through this & the symptoms sure do match what my car was doing......

I took off the jumper harness last year and played with the 10 pin harness then. I will re-check it.
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Old 06-11-2004, 02:19 PM   #9
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Sounds like your afpr may be going out. Or, you may have a fuel pump letting go on you/clogged fuel filter. Your fuel pressure should be the same from time to time with vacuum off, although I have heard that you should take the fuel pressure down to whatever pressure you want, as opposed to raising it to the pressure you want. Something about the afpr wanted to loosen up past it's set point, from what I recall.
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Old 06-11-2004, 02:30 PM   #10
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Quote:
Originally posted by 93Cobra#2771
Sounds like your afpr may be going out. Or, you may have a fuel pump letting go on you/clogged fuel filter. Your fuel pressure should be the same from time to time with vacuum off, although I have heard that you should take the fuel pressure down to whatever pressure you want, as opposed to raising it to the pressure you want. Something about the afpr wanted to loosen up past it's set point, from what I recall.
I went out this morning and the AFPR is reading what I last set it at 38-39psi. I re-connected vacuum and it dropped down to 32. With light throttle I can get the gauge to drop back to 30 psi.

A hard acceleration and it goes back to 38-39 like it should.

The AFPR *seems* to be working. I will go increase the pressure and then drop it down to 39, clear my computer and see how the drive to work is.

Like I said it didn't start until I decreased pressure from 40. After raising it up the problem continues. So maybe your suggestion will work.

I would think that the low mileage parts would be fine, but who knows
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Old 06-11-2004, 04:05 PM   #11
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I had a stumbling/stuttering problem at 1500rpm after I did some stuff to my intake and it turns out I had two of the little vacuum lines switched. Check to make sure that the four vacuum lines on th passenger side fenderwell are going to the right places.
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Old 06-11-2004, 06:53 PM   #12
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I have the same problem with my speed density 87 aod. Check your cooling system!!!! Does it do this when the car is warmed up? Or does it do it when you first start out? Please be more specific. I thought it was my ignition, fuel system, or vacuum leaks. Replaced every damn thing and still have the same problem.
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Old 06-11-2004, 11:20 PM   #13
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Quote:
Originally posted by ramjet lx
I have the same problem with my speed density 87 aod. Check your cooling system!!!! Does it do this when the car is warmed up? Or does it do it when you first start out? Please be more specific. I thought it was my ignition, fuel system, or vacuum leaks. Replaced every damn thing and still have the same problem.
Cooling system is a 3 row with electric fan and water pump with 2000 miles.

My car it runs a bit rough until it is 160+ degrees. So for a few minutes.

My problem was after the car was warmed up. It normally operates between 180 and 188*.

I upped my FP to 44 psi and then backed it down to 40 psi without vacuum.

This seems to have fixed my hesitation/ stumbling problem.

Unfortunately, my idle surge is back and will have to be tuned out via a different route then fuel pressure.
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Old 06-11-2004, 11:31 PM   #14
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bad o2 sensors cause this exact problem. make sure the car has a good tune up on it before replacing the o2 sensors. i had this exact problem & cleaned & replaced every sensor under the hood. my car is a converted 4 cyl car & we couldnt get codes out of it so it was all trial & error! after removing the upper intake & checking all the vacum lines & i replaced all the upper intake gaskets & T.B. / egr gaskets. i replaced the egr, act & tried another MAF unit. gave it two full tune ups, ignition module & added a full MSD ignition & bbk 190 lph fuel pump. no change in performance-still stuttering & bucking at a stead cruise. finally i broke down & replaced the o2 sensors & reset the comp. for 20 min. after that the car has been perfect since-2 years now- & the performance & throttle response were very noticable. gas mileage went up too! all drivability problems were GONE!
i highly recommend new o2 sensors for anyone w/ an older fox. i almost lost my mind over this! it was the best 100.00 i ever spent on my car.
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Old 06-12-2004, 12:58 AM   #15
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You might also have a bad MAF sensor. The MAF only sends a signal at idle and low RPM readings. If it is not functioning properly it can cause a poor idle quality. It will stumble and possibly backfire until the ECM senses a specific RPM Range.
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