Misfire or vacuum leak?

EDDIE KELLY

MEMBER
2001 FORD F-150
V8
4WD
AUTOMATIC
26,000 MILES

Truck began to misfire, sputter while driving, idle really rough, loss hp. So I go to auto zone, they hook up the little tester, "misfire #7". I replaced the coil, problem solved. 2 weeks later, the same symtoms happen again. Loss hp, shaking, idle rough, sputtering when I drive. So I go back to auto zone today to hook up the tester again. He said it read."lean on 6, 7, & 8. He told me I have a vacuum leak, not a bad coil. What do you suggest

2 Replies

JACOBANDNICKOLAS

MECHANIC
108,128 POSTS

Hi,

The idea that you have a lean mixture can certainly point to an engine vacuum leak. However, when that occurs, it will usually affect all cylinders and not just three.

What I suggest is this. First, check to see if you can locate any engine vacuum leaks. Here is a link that explains how this is done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If nothing is found, next check the fuel pressure to make sure it is within the manufacturer's specifications. Here is a link that explains how this is done:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Pic 1 below provides the manufacturer's specification for pressure.

If that checks good, then we need to consider a fuel injection issue. It's possible that you have partially clogged injectors in the cylinders mentioned. To check this, the best thing to do is remove the injectors and inspect their condition and operation.

Here is a link that shows how that is done:

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

I hope this helps. Let me know if you have other questions.

Take care,

Joe

See pic below.

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Friday, September 24th, 2021 AT 9:20 PM

SQM

MECHANIC
6,383 POSTS

Hello,

An engine can run lean for several reasons and vacuum leak is one of the main causes. So first step would be to do a thorough inspection of the vacuum hoses under the hood. Look for any cracked to loose hose.
You can also use crab cleaner to check for vacuum leak form the intake manifold. With the engine running spray some carburetor cleaner to the base of the intake manifold. If there is a leak the engine will idle higher till it burns off all the carburetor cleaner then go back down.

Here is a helpful article on vacuum leaks:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

You also want to make sure you have proper fuel pressure. This vehicle should have 35-45 PSI of fuel pressure with the key on and engine off.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Please let me know of any questions.
Thank you.

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