Help! My 87 stumbles under power
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Help! My 87 stumbles under power
Hey guys,
Got in my car yesterday and it started and ran fine. Let it warm up then took off. A few blocks into my drive I took off from a stop sign and, in 3rd with the car in OD (4+3) noticed there was a stumble under power–like a cylinder(s) was cutting in and out. It wasn't a one time thing it was constant as long as I was giving it gas. It stumbled in all gears and speeds. It idled fine and seemed to run good when I held the throttle steady. Once at a stoplight it seemed like it was missing–the car was rocking a little and the rpm was fluctuating showing the miss. It has never done this before. Check engine light never came on. I gave the car a tune up less than 5K ago (plugs, wires, accel cap and rotor, O2 sensor, fuel filter, K&N air filter, new fuel pressure regulator and checked tps settings). It usually runs awesome. Where should I start?
[Modified by topjimmy87, 2:17 PM 7/12/2002]
Got in my car yesterday and it started and ran fine. Let it warm up then took off. A few blocks into my drive I took off from a stop sign and, in 3rd with the car in OD (4+3) noticed there was a stumble under power–like a cylinder(s) was cutting in and out. It wasn't a one time thing it was constant as long as I was giving it gas. It stumbled in all gears and speeds. It idled fine and seemed to run good when I held the throttle steady. Once at a stoplight it seemed like it was missing–the car was rocking a little and the rpm was fluctuating showing the miss. It has never done this before. Check engine light never came on. I gave the car a tune up less than 5K ago (plugs, wires, accel cap and rotor, O2 sensor, fuel filter, K&N air filter, new fuel pressure regulator and checked tps settings). It usually runs awesome. Where should I start?
[Modified by topjimmy87, 2:17 PM 7/12/2002]
Safety Car
Joined: Nov 2001
Posts: 3,632
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Re: Help! My 87 stumbles under power (topjimmy87)
when you did the cap did you do the electronics underneath it as well ? if you did , the next thing i would think about is the fuel pump . if it is going it will seem like you are cutting out at weird intervals .
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Re: Help! My 87 stumbles under power (markmaz)
I did the cap a couple years ago and didn't touch anything else like the module or anything. When it cuts out its real short. A lot like a dead cylinder. How can I tell if the fuel pump is bad? Check pressure?
[Modified by topjimmy87, 4:13 PM 7/12/2002]
[Modified by topjimmy87, 4:13 PM 7/12/2002]
Team Owner
Joined: Mar 2000
Posts: 83,383
Likes: 87
From: Boston, MA
See if there are any burned holes in a plug wire. Might want to let it idle, and start pulling wires one by one and put them back on. If you find one that you can pull that makes no difference in idle, there ya go.
How many miles on the car? If over 100k it might be fuel pump time.
How many miles on the car? If over 100k it might be fuel pump time.
Safety Car
Joined: Nov 2001
Posts: 3,632
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Re: (topjimmy87)
with the electric pump i think it is either on or off . i would inspect the plug wires , then look under the cap . the last time i changed mine i had almost burned a hole through it . replacing it cured my miss .
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Team Owner
Joined: Mar 2000
Posts: 83,383
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From: Boston, MA
Find somebody who has a fuel pressure gauge on the end of say a 2' hose or buy one yourself. Install it on the schrader valve on the passenger side fuel rail. It is a small cap you unscrew and then screw the hose onto it. Can't miss it.
Start the car and disconnect the vacuum hose from the fuel pressure regulator. The regulator looks like a small black top-hat sitting in between the two fuel rails. You should see something like 37psi with the hose disconnected from the regulator.
Now put the hose back on, tape the gauge to your windshield, and lower your hood over it gently. Have a passenger watch the gauge as you go full throttle. The pressure should spike to 37psi (whatever value you got with the hose off of the regulator) and stay there. If it jumps to 37psi and starts to fall WHILE still at full throttle you most likely need a fuel pump. The pumps are about $75 and it is an hour swap at MOST. Very easy to do.
Also changing the fuel filter would be an excellent idea.
Start the car and disconnect the vacuum hose from the fuel pressure regulator. The regulator looks like a small black top-hat sitting in between the two fuel rails. You should see something like 37psi with the hose disconnected from the regulator.
Now put the hose back on, tape the gauge to your windshield, and lower your hood over it gently. Have a passenger watch the gauge as you go full throttle. The pressure should spike to 37psi (whatever value you got with the hose off of the regulator) and stay there. If it jumps to 37psi and starts to fall WHILE still at full throttle you most likely need a fuel pump. The pumps are about $75 and it is an hour swap at MOST. Very easy to do.
Also changing the fuel filter would be an excellent idea.
Melting Slicks
Joined: Mar 2001
Posts: 2,107
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From: Austin TX
Re: (topjimmy87)
With that many miles, it is going to go sooner or later (probably sooner at a very inconvienent time in the middle of nowhere). Very easy to do and cheap, I'd recommend it! Don't forget to do your fuel filters at the same time....
Race Director
Joined: Aug 2000
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Re: Help! My 87 stumbles under power (topjimmy87)
if you dont have one, get one of those fuel gauges that connects directly to the schrader (sp???) valve on the fuel rail. you might be running low on the fuel pressure. just went through something similar on my 85 - pump was only putting out 10psi. that sucker caused me all kinds of problems. your pump might be suppling pressure on the very low end, and it can't keep up with the demand under power. there is a screen on the pickup tube of the pump that has a tendency to also cause some issues. the newer design screens are supposed to correct this. hope this helps, and keep us informed on your findings. i wouldn't think (JMHO) its an ignition problem since it idles and starts "OK" good luck!
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Re: Help! My 87 stumbles under power (joe ciechanowski)
Yeah, I'll let you guys know what I come up with. That's one thing I noticed on this board is you find a thread with all this info then all of a sudden it ends. Well, what the heck happened?? I'll let you know.
Intermediate
Joined: Nov 2001
Posts: 44
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From: Lawton OK
Re: Help! My 87 stumbles under power (topjimmy87)
Keep us posted - - because mine just started doing the same thing after I changed my fuel filter - - I don't know if it is just coincidence or not, but I'm interested in hearing what you found your problem to be. :confused: