Engine misfiring


#1

My engine seems to misfire when I first start out and then runs smoothly. The first mechanic got a random misfire from the diagnostic and replaced all the cylinder coils which worked for about a week. The second mechanic ran a diagnostic which said random misfire and he said it was a loose spark plug. This worked for 4 days then it started misfiring again. There’s no violent shaking or backfiring. I don’t know much about cars but it seems odd that it doesn’t keep misfiring. Any ideas?


#2

At this point, I would suggest that you focus on these possibilities:

A gunked-up EGR valve
A vacuum leak
A weak fuel pump or malfunctioning fuel regulator

There may be some additional possibilities that other forum members can suggest, but most of all, you need a mechanic who is a very good diagnostician, and who will not just “throw parts”–at random–at the problem.


#3

What year and model is your car?


#4

@JanetM‌

I highly suggest you stop going to the first mechanic

He’s making guesses . . . and very expensive and incorrect ones at that

No mechanic I know would immediately replace all the coils for a P0300 random misfire

More diagnosis is required, and this guy may be unwilling or incapable of doing that


#5

Thanks people. It’a a 2006 jeep Liberty with 87,000 miles. So as I understand it, the diagnostic machine doesn’t necessarily pinpoint the trouble so it’s up to the ability of the mechanic. Are any of the suggestions - EGR valve, vacuum leak, fuel pump or regulator serious (read expensive) and am I harming the engine by running it this way?


#6

has it ever run hot?


#7

@JanetM‌

You may not like what I have to say . . .

Jeep has had severe problems with the engine in the Liberty

Often, the misfire is caused by valve problems

There is a bulletin which describes the problem. Even though it says for international markets . . . trust me, the US cars definitely had this problem.

The bulletin is 09-007-06, if anybody can find it and post it

What it amounts to is that you may very well need intake valves

I personally know somebody with a Jeep that had this very problem


#8

To answer one question, it never runs hot and after about 30 seconds (it seems less than a minute) the car runs fine even if I stop and restart the car for the rest of the day unless it’s off for a couple of hours.


#9

Could be the head gasket. Are you using any coolant (say, over a month)?


#10

If the fuel pump does not hold pressure after the engine is turned off the engine heat will vaporize the fuel in the fuel rail. When you start the engine hours later the vapor will move to the highest part of the fuel rail, the front cylinders. The engine should smooth out after 10-15 seconds.

Have the fuel pressure check, when running and after shut off.