Stumble at W-O-T
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Joined: Dec 2004
Posts: 82
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From: Kalamazoo MI
Stumble at W-O-T
Friday night when coming home from a 50 mile trip I exited the highway to a side street to my house. The traffic was heavy and required hard accelleration to blend with the traffic flow. When giving it 3/4 trottle there was a hesitation, kind-of bogged down then picked up. I thought to myself W-T-F. When I had to stop for a light I thought I would try it again, gave the car full throttle and the same thing happened, picks up for a second, bogs down then goes. The car seems to run great at part throttle, or during normal cruising. I havent had a chance to trouble shoot as I was gone the rest of the weekend. The following items will be first on the check list.
Fill the gas tank (1/4 tank showing).
Change the fuel fiter ( had the car 3-years and havent changed).
Hook up the fuel pressure gauge a take for a spin (monitor fuel pressure).
Any other things i should check?
Thanks
Fill the gas tank (1/4 tank showing).
Change the fuel fiter ( had the car 3-years and havent changed).
Hook up the fuel pressure gauge a take for a spin (monitor fuel pressure).
Any other things i should check?
Thanks
Team Owner
Joined: Oct 2000
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From: Fort Knox, KY
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Check your coil and plug wires for electrical creepy-crawlies. Also, when's the last time you changed your plugs?
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
hesitation
the only thing you move when you mash the go pedal is the throttle position sensor.
Check there first. then check the usual:
1. ignition system and plugs
2. fuel pressure
3. air filter
4. vacuum leaks
5. gas cap
6. evap solenoid
7. plugged pcv valve
8. collapsed pcv valve hose
9. oxygen sensor
If the fuel pressure gauge needle bounces all over the place at idle, there could be pressure at the pump, but low volume i.e. a worn pump, leaking check valve, etc.
Check there first. then check the usual:
1. ignition system and plugs
2. fuel pressure
3. air filter
4. vacuum leaks
5. gas cap
6. evap solenoid
7. plugged pcv valve
8. collapsed pcv valve hose
9. oxygen sensor
If the fuel pressure gauge needle bounces all over the place at idle, there could be pressure at the pump, but low volume i.e. a worn pump, leaking check valve, etc.
Thread Starter
Advanced
Joined: Dec 2004
Posts: 82
Likes: 0
From: Kalamazoo MI
I changed the plugs,wires, distributor cap and rotor 2-years ago, and have put 2,500+- miles on sense the change, but I will check again while I'm under the hood. I believe the oxygen sensor is original, and have wondered about it, I did put long tube headers on the car 2-years ago when i changed the plugs and wires. I thought when the car is at W-O-T the computor didnt look at the O2 sensor? I will check out the suggestions tonight and see what I can find.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Fuel Delivery Related 4 Sure!
Sounds like a fuel delivery problem!
1.Could be a bad TPS sensor - Measure voltage .54 -.63v with TB closed and ~5v at WOT
2.Fuel pressure problem - check for pressure at cranking and leak down times.
3.Bad gas or water in the gas - remove fuel pump assembly to siphion tank, let it air dry and add new good gas.
4.Bad or failing fuel injectors - Ohm injectors they should be around 16.8 ohms each. (When you hit the pedal to go to WOT the fuel injectors are not able to keep up with the pulse width per second request....so the car stumbles until the pulse width request is lowered/less)
1.Could be a bad TPS sensor - Measure voltage .54 -.63v with TB closed and ~5v at WOT
2.Fuel pressure problem - check for pressure at cranking and leak down times.
3.Bad gas or water in the gas - remove fuel pump assembly to siphion tank, let it air dry and add new good gas.
4.Bad or failing fuel injectors - Ohm injectors they should be around 16.8 ohms each. (When you hit the pedal to go to WOT the fuel injectors are not able to keep up with the pulse width per second request....so the car stumbles until the pulse width request is lowered/less)
Thread Starter
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Joined: Dec 2004
Posts: 82
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From: Kalamazoo MI
update
Last night when I got home I began trouble shooting and discovered the following. I hooked the fuel pressure gauge to the test port and turned the key on, the gas gauge in the car was showing that I was on reserve (it was showing a 1/4 tank when i got off the highway on Friday) I think the stumble may have been due to the pump sucking air. The fuel pressure gauge showed 42 pounds. I turned the key off, put a couple of gallons of gas in the tank and took it for a drive after it warmed up. The fuel pressure gauge showed the following during the test drive.
IDLE 35#
Light accelleration 38#
W-O-T 30#
Leak down went from 42# to 30# in a half hour after i shut off the car
The car ran good, I did several W-O-T take-off's without a stumble, except for once. I was coming out of the drive way turning and accelerating at the same time, I noticed the Gauge dropped to 10#+- during the stumble, (sucking air? I still don't have much gas in it).
Is the 30# at W-O-T normal, should it drop that much? Would a weak or failing fuel pump cause that? I will pick-up a new fuel filter and install it this weekend. Any comments?
Thanks
IDLE 35#
Light accelleration 38#
W-O-T 30#
Leak down went from 42# to 30# in a half hour after i shut off the car
The car ran good, I did several W-O-T take-off's without a stumble, except for once. I was coming out of the drive way turning and accelerating at the same time, I noticed the Gauge dropped to 10#+- during the stumble, (sucking air? I still don't have much gas in it).
Is the 30# at W-O-T normal, should it drop that much? Would a weak or failing fuel pump cause that? I will pick-up a new fuel filter and install it this weekend. Any comments?
Thanks
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Thread Starter
Advanced
Joined: Dec 2004
Posts: 82
Likes: 0
From: Kalamazoo MI
Well, I think my gas gauge may be off a little, after checking the fuel pressure and topping off the tank, the car ran fine at WOT. It has happened a couple of times since then, however it would always occur when i had around a 1/4 tank showing on the gauge, so I think the pump was sucking air. I have been keeping the tank well above a 1/4 tank since then, and have not had a problem. I still don't know if 30# fuel pressure at WOT is OK, it sure seems like it should be higher.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
What you experienced, especially making a turn or rounding a corner with a low tank has been experienced and reported by many members here.
Regarding the pressure, you're probably OK, my 87 dropped to about 35-37 if I remember correctly, at any rate the fuel pressure test you posted was absolutely fine, especially the time and how far it dropped off. It certainly wouldn't hurt to change that fuel filter....probably do.
As far as the 02 sensor, unless your mileage has dropped significantly or an 02 code popped up....I wouldn't worry about it.
Regarding the pressure, you're probably OK, my 87 dropped to about 35-37 if I remember correctly, at any rate the fuel pressure test you posted was absolutely fine, especially the time and how far it dropped off. It certainly wouldn't hurt to change that fuel filter....probably do.
As far as the 02 sensor, unless your mileage has dropped significantly or an 02 code popped up....I wouldn't worry about it.
Thread Starter
Advanced
Joined: Dec 2004
Posts: 82
Likes: 0
From: Kalamazoo MI
What you experienced, especially making a turn or rounding a corner with a low tank has been experienced and reported by many members here.
Regarding the pressure, you're probably OK, my 87 dropped to about 35-37 if I remember correctly, at any rate the fuel pressure test you posted was absolutely fine, especially the time and how far it dropped off. It certainly wouldn't hurt to change that fuel filter....probably do.
As far as the 02 sensor, unless your mileage has dropped significantly or an 02 code popped up....I wouldn't worry about it.
Regarding the pressure, you're probably OK, my 87 dropped to about 35-37 if I remember correctly, at any rate the fuel pressure test you posted was absolutely fine, especially the time and how far it dropped off. It certainly wouldn't hurt to change that fuel filter....probably do.
As far as the 02 sensor, unless your mileage has dropped significantly or an 02 code popped up....I wouldn't worry about it.
