Hi, new member here.
I have a 98XLT, 4.2L V-6, 113,000 miles, and I'm having a problem with an apparent misfire or stumble at around 1500 - 1800 rpm. This happens after I reach cruising speed on the highway. It idles fine, and there is no problem accelerating from a stopped position. When it happens, the tach bounces up and down by around 300 rpm each way, and if I give it some gas, it stops stumbling. I replaced the spark plugs and it ran fine for about a week, and now it's doing it again worse than ever. I was sure it was a bad plug because it stopped stumbling for a while right after I changed them, but I guess that wasn't it.

I have been getting an on and off Check Engine Light for a while even before I noticed the engine stumble problem. I'm getting 2 trouble codes: P0402 (EGR Flow Excessive) and P0117 (Engine coolant temperature sensor circuit low input).

I noticed from other threads that this misfire or stumble could be related to the P0402 code, but I'm not sure how (or if) the P0117 code fits into this picture.

Thanks for any help.

 

It sounds like your DPFE sensor.Its that little rectangle box,about 3x2 inches,mounted to the left of the EGR,with vacuum lines running to and from it.You can take it to any parts store and they have a procedure to check them.It cost $50.00 for the sensor.

 

Thanks. I located that sensor, but I was wondering, if I remove the sensor to bring it somewhere to get tested, can I drive it with the sensor removed?

Also, I did some research on the P0117 code, and I found that it's possibly a bad Engine Coolant Temperature Sensor. Problem is I can't find where it is located. I have the Haynes repair manual, but I think the picture which shows this sensor is for the V-8 engine. Anybody know where this sensor is on the V-6?

Thanks.

 

It's on the drivers side tucked away behind some hoses. It's hard to see but dig around and you should be able to get it.

 

Looking at the front of the engine,just to the right of the t-stat housing,you will see two sensors.The one in a upright position,closer to the t-stat housing,is the coolant temperature sending unit.The one at about a 45* degree angle, just to the right of the coolant sending unit,is the coolant temperature sensor for the computer.
If you plug the vacuum lines,I dont see any harm in driving it.When I took mine to have it checked,I drove my truck to Autozone then removed the sensor in the parking lot.Took about 3 minutes.

:santa:

 

Thanks, I can see it now. Looks like it's going to be fun trying to get a wrench on that thing. I guess I'll start with the DPFE sensor and see if that fixes at least one of the trouble codes.

I'm still trying to find out if both of these trouble codes are linked somehow.

 

Update on this problem: I replaced the dpfe sensor, and I believe this fixed the check engine light problem. It's been about 2 weeks and no light. Both trouble codes have stopped, so I guess the dpfe sensor was causing the engine coolant temp. code as well.

I'm still getting the hesitation, although not as severe as before. I must have another issue which is causing this. I'm wondering if it could be a bad spark plug wire. I'm using original wires with 114,000 miles, so I guess it's overdue for a change.

 

Well I put new wires in and the engine is still stumbling at highway speeds under light load. This is getting frustrating. So far I've replaced plugs, wires, fuel filter, DPFE sensor, air filter, and I gave it the seafoam treatment (through the brake booster vacuum hose).

There has been no Check Engine Light since replacing the DPFE sensor.

I'm thinking of trying a new coil pack next. I'm not sure what else to try

 

Check the pcv hose where it attaches to the upper intake for any vacuum leaks. It will take a while for these to produce a check engine light. If you have access to a scanner than can look at PID data, check the short term fuel trim. It should be less than +10%. If it is sucking air, it will be +20 - +30%. This indicates an air leak. The oxygen sensors will also indicate lean. Codes P0171 and P0174 are sure signs of an air leak. The air leak will cause spark knock on accel and an off idle stumble.

Robbie

 

This sounds exactly like the same problem that I and another member on here are having. I too have done everything you have with the exception of the DPFE sensor.... I'm stumped....

 

PCV valve and hose appear ok. I do have a scanner, but I think it can only display codes. I don't see anything on it about displaying PID data.

I'm starting to wonder if this is actually a transmission problem, since it only happens when in overdrive.

 

Check fuel pressure. Could be a weak fuel pump, too. Should have 35psi at idle.

Robbie

 

I haven't checked that yet. I assume you check the fuel pressure at that shrader valve. Do you check the pressure while the engine is running?

Also, I think I can rule out a transmission problem, since I noticed that it also stumbles in 3rd gear as well as OD.

 

You are correct on both counts.

Robbie