s99

Hi,

Im a new owner of a 2000 Jeep Grand Cherokee Laredo 2WD, 4.7, 178k, rebuilt HG with some issues, and if you're familiar with any of them your help with be extremely appreciated!

1. I need to give the car some gas when starting on a cold engine or else the engine won't pick up. The RPMs will drop and turn off immediately after startup. I have to rev the engine for several seconds before it idles by itself. During a warm engine the RPMs won't drop after startup, however I still have to give it some gas.

2. When the car is on a loud hissing sound is heard near the brake pedal, under the dash. The hissing sound reduces when the RPMs go up and while pressing the brakes. I temporarily removed the pcv vacuum hose from the brake booster and the hissing went away. I thought this vacuum leak was the cause of my third problem:

3. While driving or idle (idles great), the engine sputters as I slowly press the gas, eventually the car would kick and take off normally if I press the gas a bit more. The car shifts and drives fine if I don't let go of the gas. The worst part is that the engine stalls EVERY TIME I press the brakes hard (like taking an exit off a highway). After the car shuts off the hissing sound slowly fades away. I temporarily blocked the pcv vacuum hose from the brake booster while idle, the hissing went away and the sound of the engine changed slightly but the sputter continues when I rev the engine, I may still hear a very slight hissing sound around the engine, I'm not very sure.

I know the Brake Booster needs replacement. I love this car and I hate to hear it sputter like that. Im not driving it at the moment and the battery has been drained. I won't have time to work on it until next week. Please let me know what your thoughts are, your time is extremely appreciated!

Here is a video, sorry about the horrific sound quality.

dave1123

Welcome to CF.

You obviously know about the brake booster. Have you checked for OTHER vacuum leaks? Check the CCV valve, the MAP sensor hose, the vapor canister purge hose, and any other vacuum lines you can find.

It is also possible the plenum chamber gasket is leaking. That's the tough one to check because it's on the bottom of the plenum. The reason I suspect a vacuum leak is because your engine is idling a little high. There is a "smoke" test that can find hidden leaks. I'm sure someone else on the forum can tell you about it better than I.

What is the HG you refer to?

s99

Thank you for the response!

My engine is idling at 1100 RPMS and drops slightly when the brakes are pushed and the booster vacuum hose is removed. There must be another leak somewhere that causes that. I was able to locate all PCV hoses, blow into them and they all have a tight seal/block (except for the booster).

The person who sold me the Jeep disconnected the check engine light bulb along with resetting the codes. He told me there was nothing wrong with the engine, no engine light and the sputtering was caused by the vacuum leak. A week later, I decided to check for codes anyways, I turned key to ignition 3 times and 6 codes came up. (lesson learned from craigslist :/)

Codes read:
P0351 - Ignition Coil #1 Circuit Malfunction
P0352 - Ignition Coil #2 Circuit Malfunction
P0355 - Ignition Coil #5 Circuit Malfunction
P0356 - Ignition Coil #6 Circuit Malfunction
P0357 - Ignition Coil #7 Circuit Malfunction
P0358 - Ignition Coil #8 Circuit Malfunction

My neighbor who owned 10 Jeeps and heard the sputtering next door said he it sounded like a fuel rack problem.

Again, thanks for your help and any advice is still appreciated.

dave1123

Sounds like you have an ignition coil supply circuit problem. It would be unusual for 6 coils to be bad.

I've never been in the presence of the illustrious 4.7 so I couldn't tell you much about it and the Haynes manual isn't much help. Try checking the fuel pressure while it's running. Several auto stores have a guage for rent. I know the 5.2 and 5.9 have a fuel pressure regulator on the back of the throttle body. The 4.7 is a mystery. The other V8s are TBI where the 4.7 is port injected. If you have good running fuel pressure, I would trust the PCM trouble codes and look at the ignition circuit.