Sensors and Codes...

1996 Jeep Grand Cherokee 4.0 6cyl 110K
(New CPS, TPS, IAC, CAT Converter, Coil, wires, cap, plugs, batt/alt good)

Just a little informative thing here that may be good for everyone, but especially me who is just beginning at "fixing" my own Jeep. Looking for opinions from those who have been doing this for quite a long time, I have a question about sensors and "codes" in general.

In most cases, when the sensors actually start to go bad or fail, are you likely to get a CE light (and code for that sensor) or information that can be read in the computer that shows abnormal operation?

The reason I ask is because I read so many posts about replacing sensors and not many of them refer back to actually diagnosing a "code" to point you in that direction.

Many sensors such as the MAP, TPS, CPS, IAC, O2, etc have been referenced as causing a myriad of problems when they begin to fail, but do they eventually throw a "code" if they go long enough.

I have been battling a "Mult Cyl Random Misfire" problem for some time and I have had a terrible time tracing this down. At this point, the only sensor I haven't replaced or checked is the MAP, but I am not getting anything but a random mult cylinder misfire code every time the CE light illuminates...In my case, I am also not getting any information loaded into the PCM that points to a particular sensor either (Checked at the dealership by a mechanic I know). No missing voltages or variations recorded at all.

Thanks for the help in advance.

Bryan

In most cases, when the sensors actually start to go bad or fail, are you likely to get a CE light (and code for that sensor) or information that can be read in the computer that shows abnormal operation?

Many sensors such as the MAP, TPS, CPS, IAC, O2, etc have been referenced as causing a myriad of problems when they begin to fail, but do they eventually throw a "code" if they go long enough.


Not all failures will get the check engine light. I had an issue a few weeks ago, no check engine light, but had the code for the crank position sensor. So it's possible to throw a code without the check engine light. (2000 4.0)


The reason I ask is because I read so many posts about replacing sensors and not many of them refer back to actually diagnosing a "code" to point you in that direction.

There is a lot of experience on this forum(not me!), and from the problem comes the solution, since the same problems have occurred multiple times.

I know you probably have seen this but anyway:

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?s may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in ?Limp-In? mode.

The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

This condition may occur when the engine is not allowed to run at engine RPM?s that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM?s and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve.

Yes....in fact I have seen that and it fits quite well with my problem.

I have attempted to improve/fix it with MOPAR combustion chamber cleaner, but maybe I need to clean the crap out if it a little more with a couple more cans first. I've done it once and let it sit for 30 minutes before starting it up again.

Riddle me this.....

Someone mentioned spraying the MOPAR combustion cleaner in the throttle intake as instructed, but letting it sit for and hour or more on a hot engine to really work on it good.

Will this hurt anything if I let it sit for a hour, hour-half, two hours before blowing the stuff out of my exhaust????

Bryan

Found this on a Web site: It may be helpful...Steve

From : Greg at Chrysler
Mopar Combustion Chamber Cleaner/Conditioner is really good for removing carbon deposits, but the directions on the can are not so useful. The following directions outline the way that professional mechanics have been using the cleaner for years -- they were finally published in TSB 18-31-97 for 1996-98 Jeep 4.0 Liter misfire conditions:
decarbonizing Procedure
1. Operate the vehicle until the vehicle reaches operating temperature.
2. Remove the air tube from the throttle body.
3. With the engine at an idle, spray the entire contents of Mopar Combustion Cleaner, p/n 04318001, directly into the throttle body. Allow the vehicle to load up with the cleaner to the point of almost stalling out.
4. Shut the engine OFF after the entire can is ingested.
5. With the hood closed and the vehicle parked inside the garage, allow the vehicle to soak for two to three hours. This will ensure that the engine will maintain its temperature and will allow proper solvent penetration.
6. Drive the vehicle on a highway/freeway that will allow the vehicle to be driven safely at the posted speed limit. Upon entering the highway/freeway, accelerate hard to the posted speed limit and maintain speed. Slow down and then perform 5 to 10 Wide Open Throttle (WOT) upshifts. Continue driving at the maximum speed limit for 1-2 miles (if conditions allow).

Greg Smith

Thank you.

I thought I had read that somewhere before, but could not remember the details. I'm going to give that a shot this weekend and see if it helps my situation. I just don't seem to be logging any diagnostics to even hint at a sensor except "random misfire" codes that bounce around with no pattern.

Some days it runs great, some days it doesn't. However, it does appear that since I did it the first time (30 minute wait) it has been a little smoother on the highway and the problems occur more from a dead stop acceleration now.

Checked my TPS (Wells brand only a couple of months old) today and that is measuring at 3.7 VDC at WOT, but I am not sure that is really a "bad" reading. I have seen others with that same measurement here. The also checked the MAP today too and that is good. The IAC is brand new. The CPS is new. The coil is new.

My only other thought is a vacuum leak, but I have not been able to isolate anything there yet. It sounds like something is "hissing" but it is very hard to tell over the fan noise and I don't have a gauge. My idle symptoms seem to point in that direction too.

We'll see...I'm still determined to figure it out one way or another. If nothing else, I have sure have learned my way around my Jeep. Probably gonna buy a 2005 or 2006 this summer anyway, but I want the satisfaction of fixing this thing first. :0)

Thank you again.

Bryan