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2003 KJ Liberty 3.7L gas - hard to fix misfires, need suggestions

2K views 11 replies 5 participants last post by  kueh 
#1 ·
Note, this vehicle is currently driven very little, mostly because of this issue -- less than 3k mi in the 15mos, and maybe only 500mi since Halloween 2016.

HELP! I've fighting this 2003 Liberty 3.7 gas with ~150k miles now for the better part of a 9 mos, both myself and with the help of two shops, and so far no lasting progress. Basically each time we make a fix, the misfires comeback within at most a couple weeks. There have been no codes observed other than misfires. We live at 7800ft altitude, with a 1800ft elevation interstate drive from town. Between replacing the plugs and coils, and the MAP, I barely made it home one day because the engine was running so roughly and wouldn't accelerate past 35mph. After the MAP, it ran beautifully for 1 month, then back to general roughness esp at idle, misfire codes

What else am I looking for here? This is driving us batty! One other shop suggested a full head-gasket both sides with machining which is $2k+, but with no apparent compression problems that seems wrong. Another shop suggested maybe a plugged CAT causing back pressure ... does this even make sense in modern CATs (I thought this was an 80's and 90's problem that rarely exists today).

What has been done:
- compression test - all cylinders similar and OK
- all new plugs, factory OEM, gapped
- all new ignition coils
- new MAP sensor
- new muffler (not related, but fyi)

From my ODB2 scanner (these are the only ones I recorded), there were more instances
Nov 02 2016 - P0300, P0303, P0305
Dec 09 2016 - P0300, P0302, P0303
Jan 14 2017 - P0300, P0301, P0302, P0303, P0305
 
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#3 ·
(I know, late reply)

OK ... just had muffler shop check for back-pressure -- nothing.

What is really the diff between leak-down test and compression test? What will it tell us, and how much (approx) should I expect to pay for that? Will that really point to the solution?

This one is still killing us with the nearly undrivable condition and no fix. :confused:
 
#5 ·
When the problem happens, hook up a vacuum gauge to the vacuum hose that comes off the brake booster to the engine. Do not go thru the check valve. Even with misfires, the needle should be rock steady. If it flutters, you have a mechanical issue. What shops have you taken it to exactly?
 
#10 ·
You check the fuel pressure at the Schrader valve on the fuel rail. You may be able to borrow a pressure tester kit from a local auto parts loan-a-tool program. It's just a pressure gauge with adapters to fit the valve. Not positive but I think it should be around 50-60 psi. If low you could be lean which could cause some misfire issues.
 
#7 ·
OK ... well, the problem always happens, so that should be easy. Can you rent vacuum Gauge easily or ...?

Regarding specific shops ... not sure what you are looking for. I don't think it would be appropriate to name the shops in a public forum.
1) long time shop in Golden CO - three different times, including one time they had it during a month-long trip. All they recommended was a $2500 head-gasket replacement (this is a place that I used for many years on multiple vehicle, been in business since the 70's or 80's)
2) Denver-area mobile mechanic who did the compression test, and replaced the plugs and coils
3) Golden-area muffler shop that checked for back pressure
4) Golden-area mechanic recommended by #3 who after hearing the issue and discussing, declined to work on the vehicle
5) I did the MAP valve myself (and a PCV valve)
 
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