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Slightly Rough Idol after Fuel Pump Replacement

Old 06-19-2018, 01:48 PM
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Default Slightly Rough Idol after Fuel Pump Replacement

Good afternoon. I have a very solid running 97 LT1, the car and engine only have 25k miles. The fuel pump failed and was replaced a couple months ago. Ever since, I've noticed the car idols rougher than it used to. To me, it feels like it's misfiring every now and then. Nothing too harsh, it keeps running, just feels like it's skipping a tiny bit.

I have had it hooked up to the shop's scan tool computer, but they say it doesn't register any misfires, and there's no other clues as to what might be happening. There's no misfires that I can tell at other engine speeds above idol, it still runs very solid. My first thought was fuel injectors, but I'm not really sure how to troubleshoot them. I ran a couple cans of SeaFoam through it to see if I could even notice a difference, but no change that I could tell.

Short of pulling all of the injectors and having them tested or replaced, any thoughts on something else I can try? I should point out, it happens whether the engine is cold or warm. Thanks for any/all help!
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Old 06-19-2018, 01:56 PM
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What's your fuel pressure?
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Old 06-19-2018, 02:03 PM
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Good question. When I had the pump replaced, I made the assumption the shop checked the pressure afterwords, but maybe not. Being mostly stock, what should I see for pressure?
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Old 06-19-2018, 02:11 PM
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Nevermind, sorry for the stupid question. I did a quick look, I see it should be 43-44 all the way through the RPM range. I'll check it out later today. Thanks.
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Old 06-19-2018, 02:29 PM
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in order to check check that kind of pressure you must take the vacuum off the fuel pressure regulator. check it with and without.
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Old 06-19-2018, 03:01 PM
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Hope I'm not asking too stupid of a question, but could you describe that procedure? Do I disconnect the vacuum from the regulator before testing the pump with the engine off? Do I reconnect it to watch the pressure while engine is running?
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Old 06-19-2018, 03:36 PM
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you check fuel pressure with the vacuum line off, and plug that vac line, with the engine idling. it should be 43.5 psi

Put vac line back on and the pressure will drop around 7 psi (normal) then drive around where you can have a good "sustained" WOT pull in any gear. The fuel pressure should raise some...never drop

what could have happened when your pump was replaced is debris could have gotten in fuel line, past the fuel filter, and partially clogged one or more injectors. A 20+ year old car with 25k mi on it suggests it sits most of the time. Fuel system can develop crud that gums up stuff like injectors

injectors can be easily serviced. I use www.cruzinperformance.com
$15 per injector IIRC
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Old 06-22-2018, 01:39 PM
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Thanks for all the info. I'll be getting the injectors tested/serviced. Just curious, is it possible for a bad fuel pump to cause any damage to injectors, I made the mistake of letting the car try to run while the pump was dying, and it was scary, super rough, shaking itself to death. Just curious if trying to shoot fuel and shooting air can damage the moving part of the injector.
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Old 06-22-2018, 02:06 PM
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a failing fuel pump won't damage injectors
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Old 08-02-2018, 09:55 PM
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Good evening. Looking for opinions on this one... As the post stated, I have a low mileage LT1 (30k) that had a failed fuel pump recently. Since the fuel pump replacement, I've had a slightly rougher idle than usual. I have yet to get the injectors tested/cleaned, but I did test the fuel pressure. I know it's not right, but here's what I got:

Initial Pressure when pump is running when ignition is turn on = ~43psi
Pressure While ignition on after pump stops = ~41psi
Pressure After car has sat for about 10 minutes = ~40psi
Pressure while car started and idling (and running at normal speeds) = ~33psi
Pressure while rev'ing = ~37psi

I've read that these numbers are pretty low, so I suspect that's the cause of my rough idle. I was curious what the opinions are on the common cause, whether it would be regulator or pump. I also wondered if maybe the answer is the regulator, is that what could have caused my pump to die at such a young age. But that's just a theory. Any thoughts greatly appreciated!
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Old 08-03-2018, 10:57 AM
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Originally Posted by 97V8Bird View Post
Pressure while car started and idling (and running at normal speeds) = ~33psi
Pressure while rev'ing = ~37psi

. Any thoughts greatly appreciated!
the other #'s are fine...but 33 psi at idle and normal driving is low. the 37 during rev is low but not real low. It should go up to about 40. This assumes the FP gauge you are using is accurate

Run engine, shut off, pull vacuum nipple off FPR...is there any sign of fuel or fuel oder in the rubber nipple? If so FPR is bad

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Old 08-03-2018, 11:32 AM
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Originally Posted by 97V8Bird View Post
Pressure while car started and idling (and running at normal speeds) = ~33psi
Pressure while rev'ing = ~37psi
Revving how? In neutral or while the engine is under load at WOT?
The simplest way to similulate WOT is to remove the vacuum line from the fuel pressure regulator. The numbers you listed here do not mean anything unless these are values with that line removed.
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Old 08-10-2018, 03:25 PM
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Thanks again for the advice, and sorry for wasting your time without properly testing things first. I did pull the vacuum line and pressure looks great at all RPMs.

I also had the Fuel Injectors professionally cleaned and tested. They were already in good shape also.

So I'm still left with the mystery of the slightly rough idol. I have no idea what to look at next. If it's not the fuel pump or the injectors, and my shop didn't find any vacuum leaks. It's not horribly rough, just is obvious to me that somethings not right. If you were standing behind it, you would think it was cammed more than it is. The shop's OBD2 computer says it doesn't show any misfires logged, but I plan to look a little deeper into that. Other than that though, any other idea what could lead to a rough idol on a very solidly running low mileage engine? Thanks.
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Old 08-13-2018, 05:40 PM
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Are there any tricks to figuring out if a slight misfire at idol could be ignition or fuel related?
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