Multi Cylinder Missfire?

[bruce]

Ok all you mechanics out there. . . . I have a 1997 Jeep Grand Cherokee 6 cylinder that I keep getting a Code 300 with this code I also get a 301,303 or other cylinders but only 2 codes. I replaced the wires, plugs, cap and rotor and still get this code set (only thing not replaced in the ignition is the coil).

It sets the code while the engine is underload going up a hill/slight grade, the engine temp climbs which leads me to think the engine is going lean? The engine has a serious drop in power, I changed my gas to high test and this seemed to help some.

Yesterday I was driving and the engine acted up again (slight grade, part-time 4x4 engaged) I checked the codes today and had a 300,301,302,303,304 and 305 missfire. I've searched all over the net for an answer and it seems nobody has an answer on the code 300's?

Any ideas? I'm thinking coil now since it set codes for all but one cylinder? Am I going in the wrong direction? Could it be fuel related since the temp climbs? Daily running it is fine, gas mileage is normal but get it under part throttle load on a grade/hill and the engine SUCKS!

It sets no other codes besides the 300's

[1960flh]

Bruce;

What does this application use for a timming trigger, it could be retarding itself, (lack of power, heat build up).

Just thinking.

Rick

[gofaster]

Coils work or do nothing, if it runs it's not the coil.

[Tony]

The ECU has obviously detected some sort of fault, and it only has inputs from all the various engine sensors to work from. Something like a high voltage ignition misfire, or fuel starvation causing lean running, the ECU would never know about. So I think you can ignore things like coils and fuel pumps.

It can really only be an input from one of the engine sensors going out of range. There is no way to guess, you need to discover what causes the 300 series of error codes. It could be going lean, or extremely retarded, but whatever it is, the ECU is probably doing it because it is confused by one of the engine sensor inputs being way off what it expects to see.

The Jeep factory workshop manual should have a list of error codes and what they all mean. Go and see the head grease monkey at your nearest large Ford dealer and ask, he will either know, or be able to find out what 300 means.

[2seater]

Is this a single coil or a multiple coil pack? Port fuel injection, right? Light to moderate load at low rpm would likely get a misfire if lean. Weak spark under the same conditions will misfire too.

[bruce]

Single coil, port fuel injection. Without it setting any other codes to other sensors it seems hard to diagnose the 300 codes. I've read alot of forums with people like myself having code 300's and not getting any sensor codes set. Still makes me think a weak coil.



Edited By bruce on 1171852164

[sirknightbighead]

Is there a leak at the exaust manifold or intake gasket? Is the exhaust manifold cracked(like mine did and a lot of others for that matter)? I had a 96 Jeep Grand Cherokee. I have seen cracked manifolds and leaking intake/manifold gaskets throw those codes, especilly under a load. It may not seem important but the order those codes set could help diagnose which cylinders are the main problem. IE P0302 first followed by P0303 could mean that those cylinders are the main fualt and the others are just coding due to the misfire condition. If the gaskets are leaking or the manifold, it can fool the O2 sensor readings to think it is lean, when all it is actually doing is seeing the extra air in the O2 reading and richening the mix when it doesn't need to. Or a leak at the intake would lean out that cylinder. The timing sensor for that vihicle is on top of the bellhousing of the transmission and works off of the flywheel. Hope that helps somewhat.

[bruce]

I replaced the exhaust manifold a few years ago with a new one with the bellows (mine was cracked). I picked up a new coil today and will replace that. (didn't cost all that much, so it is worth a try)

While under the hood I'll take a look-see for any leaks around the manifold. I only have a scan tool so not sure if I can find the order the codes are set? If it would set a code for one of the sensors it would make it alot easier to find the problem but just setting the 300's makes it a "search for what's wrong".

I have not looked at the major things that could be wrong ie leaking valves, timing chain etc and would like to put those fears out of my mind

Although it does have 160,000+ miles on it . . . THANKS FOR EVERYONE'S INPUT!!



Edited By bruce on 1171903565

[1960flh]

[color=#000000]Bruce;

In reading the posts the other thing that comes to mind is injector issues carbon etc. I found this post of a tsb not sure but it sounds like your issue.

*******
0900303 - 4.0L MULTIPLE CYLINDER MISFIRE
Date: 08/01/03

Model Year(s): 1999-2004

Description: NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE. This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary.

Details: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 50 - 70 MPH and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0 C (32 F).

If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC

[bruce]

Well that is my problem to a "T"!!

[1960flh]

Bruce;

Be carefull on any decorbonizing you do as it can have an adverse effect on HEGO/EGO sensors. I might try one of the snake oil treatments at first with a couple hard runs. If no improvement remove the EGO's (if the system will let you) before any decarbonization through the intake.

[bruce]

New coil installed and I'll kick the overdrive off and run some highway miles today taking my daughter back to college. I had been driving 80-90 miles a day but in the past 2 years most of my driving now is less than 10 miles a day so that info makes sense.

This inline 6 has been one of the best running engines I have every had, it never revs more than 2000 rpms even at highway speeds. But like I said it does have some miles on it.

How great is it that there is a bunch of mechanics with a very diverse background to help diagnois a problem here on the forum?

If anyone else ever has a problem feel free to ask away here in the general BS area!