98 gt surging under acceleration

stang98gt

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Oct 1, 2011

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Columbus, Ohio

Oct 1, 2011

#1

• Oct 1, 2011

• #1

i recently bought my 98 mustang gt with the 4.6L about 3 weeks. after 1 week of owning it a check engine light came on and i had to change the o2 sensor. last week i had to change the valve cover gaskets and spark plugs after finding oil on a spark plug. since then my car has been surging under immediate acceleration. its not as bad when the car is cold. it acts its worse when in 5th gear in between 50-60mph. once i get into the higher rpm range 2k-2500k its fine. i have no idea what the problem is and im at my whits end. lol any help would be greatly appreciated.
only mods are: MAC cold air intake, Magnaflow cat-back exhaust
p.s.- it idles and starts just fine, it does however drop rpms when coming to a stop after warmed up

 

wmburns

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Aug 14, 2009

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#2

• Oct 1, 2011

• #2

The #1 casue for bucking is a weak ignition (spark). Next is a vacuum leak. 3rd is excessive EGR flow.

 

stang98gt

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Oct 1, 2011

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Oct 1, 2011

#3

• Oct 1, 2011

• #3

is there an easier way to find a vacuum leak other than spraying carb cleaner on all the vacuum lines? my guess was a vacuum leak as well, but i have yet to find one. i just replaced fuel filter today, didnt help, but it needed replaced anyways. no engine light has come on at all. what causes excessive egr flow?

 

wmburns

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#4

• Oct 1, 2011

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A propane based vacuum leak detector is an option to an expensive smoke tester.

Thexton 382 Vacuum Leak Detector - Stores and Prices

I personally think it's a mistake to use carb/choke cleaner to find vacuum leaks. Why? Because the harsh cleaners can damage gaskets thus trading one problem for another.

Consider that there are several possible causes for low/slow idle.

• Vacuum leak between the MAF and TB butterfly.
• bad/clogged/slow acting IAC value
• poor cylinder power balance. IE one or more cylinders not pulling their weight.

For more information on trouble shooting IAC/idle problems:
Is your mustang stumbling, dieing at idle or idling rough? - Page 14 - Ford Mustang Forums

As for possible causes of excessive EGR flow:

• Vacuum leaks to/from DPFE sensor
• bad EGR valve
• bad DPFE sensor

If asked to handicap this thread, the common possible cause is weak ignition (spark). However, it might be wise to check the base motor's health with a compression test.

Also possible there are two problems. Excessive EGR flow at cruise RPM's and a separate problem causing the slow idle.

 

stang98gt

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Oct 1, 2011

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Columbus, Ohio

Oct 1, 2011

#5

• Oct 1, 2011

• #5

my idle is fine. just bucks when pressing the accelerator quickly. thank you for all your help thus far. i am going to look for a vacuum leak again tomorrow as well as check all plugs and wires.

 

stang98gt

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Oct 1, 2011

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Oct 2, 2011

#6

• Oct 2, 2011

• #6

so i changed the fuel filter yesterday as well as run fuel system cleaner through the car, didnt help with the bucking. still cant find a vacuum leak. any other advice as to what it could be???

 

wmburns

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Oct 2, 2011

#7

• Oct 2, 2011

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Consider a fuel injector cleaning service such as injectorrx.com. Once cleaned and flow tested, the injectors are as good as new. Cheaper than new.

Confirm that the fuel pressure regulator intake vacuum reference line is connected and leak free.

What is the fuel pressure with the reference line disconnected at idle?

Did you change the PCV valve?

Are you still seeing oil in the spark plugs?

What are the results of a compression test?

What have you done to rule out a clogged exhaust?

How many miles on the motor?

 

1987stangman

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Jul 12, 2006

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Oct 2, 2011

#8

• Oct 2, 2011

• #8

wmburns said:

Consider a fuel injector cleaning service such as injectorrx.com. Once cleaned and flow tested, the injectors are as good as new. Cheaper than new.

Confirm that the fuel pressure regulator intake vacuum reference line is connected and leak free.

What is the fuel pressure with the reference line disconnected at idle?

Did you change the PCV valve?

Are you still seeing oil in the spark plugs?

What are the results of a compression test?

What have you done to rule out a clogged exhaust?

How many miles on the motor?

Click to expand...

Burns, I think that you hit the nail first reply. If this issue did not exist prior to removing and installing the plugs could be a bad plug wire(s). We all know that removing these wire can cause issue if they are old. His issue really sounds plug wire or coil pack related.

Second, is the oil concentrated on the top (as if it dripped from a cam cover) or is it on the combustion chamber side (valve seal,rings). As an example; My old NPI engine in my 96' had a oilly plug on cyl 8 due to valve seal problem. This however, did not cause any issues for many miles between changes.

Any additional codes being thrown?

 

stang98gt

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Columbus, Ohio

Oct 2, 2011

#9

• Oct 2, 2011

• #9

oil was on cylinder #1, i checked all the spark plugs today and they are clean. wires looked fine, but i plan on getting new ones this week as well as pcv valve. not trying to rule anything out. car has 77,500 miles on it. also, i havent done a compression test, not even sure how to?? and no codes have been thrown.

 

stang98gt

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Oct 1, 2011

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Columbus, Ohio

Oct 4, 2011

#10

• Oct 4, 2011

• #10

UPDATE: changed spark plug wires...didnt change anything. also, i am now hearing a faint high pitched whistle that comes and goes????

 

stang98gt

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Oct 1, 2011

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Columbus, Ohio

Oct 4, 2011

#11

• Oct 4, 2011

• #11

UPDATE: changed spark plug wires...didnt change anything. also, i am now hearing a faint high pitched whistle that comes and goes???? please any suggestions would be great!

 

wmburns

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Oct 4, 2011

#12

• Oct 4, 2011

• #12

stang98gt said:

UPDATE: changed spark plug wires...didnt change anything. also, i am now hearing a faint high pitched whistle that comes and goes???? please any suggestions would be great!

Click to expand...

Bad IAC valve or other vacuum leak.