'98 Dakota 3.9L Misfires on Bank 1

63vette

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#1
I'm working on a 1998 Dodge Dakota Ext Cab 4WD with a 3. 9L. The CEL light comes on occasionally (averages about once per month). The codes are almost always P0300, 301, 303 & 305. About 6 months ago, I replaced the plugs, wires, cap & rotor. It was fine for about a month and then the light came back on with the same codes. The freeze frame data is usually about the same: Load 40% range; Coolant temp 150 to 195; RPM's between 2500 to almost 3000; MAP 23 in. ; LTFT low %; STFT ~ 1%; MPH 40 to 55.

I have poured over the wiring diagrams thinking that there might be something in the injector wiring that is unique to the cylinders on the left bank and there isn't. I thought there might be a vacuum leak at the intake on that side so I checked the torque. The bolts on the right side were at least at the specified torque setting but the ones on the right side - especially in the center - turned about a turn or so before hitting the torque setting. Some weeks later, the truck set the CEL again.

I did a road test with my (AutoEnguinuity with enhanced support) PC scan tool and was able to monitor the misfire counts actively. There weren't enough to set the CEL but the three problem cylinders would ring up counts in the mid teens to low 20's - usually under a pretty good load. Everything looks good at lower RPM's.

I've checked the TSB's and can't find anything related. Has anyone run into misfire issues with the 3. 9L on one bank and not the other?

Thank you in advance. . .
 

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#2
Looks like you are doing the right stuff. Have you checked compression, often overlooked. Have you checked fuel pressure, I have had a pump cause misfires on only some cylinders. Some injectors need more fuel as they get older. What brand plugs did you use, these guys ARE sensitive to brand. Post back, Transman
 

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Transman, thank you so much for the response. No, I haven't checked compression. I was talking to my oldest son who is a mechanic at a Dodge dealer the other night and he thought that doing a compression or cylinder leakage test would be a waste of time. He's thinking that there is carbon in the combustion chambers and on the valves that causes them to stick when the engine is hot and at (relatively) higher RPM's. Like he said (which made sense at the time), cranking the engine for a compression check doesn't even come close to idle/lower RPM's where the engine doesn't indicate any misfires.

I can't remember if I have checked fuel pressure - I've had this truck in so many times - but I don't think so.
I was thinking that if there was a fuel pressure problem, it wouldn't affect just the cylinders on one bank but it does make sense that if the fuel pressure/volume is marginal, that at higher RPM's the injectors may not be getting sufficient fuel. I wish I had a one of those wireless gauges so I could monitor the fuel pressure on a road test - under load!

I put in Champion 436/RC12LC4 standard plugs. I almost always use Champions in Chryslers, AC's in GM's, Autolite or Motorcraft in Ford's and NGK's in most everything else...Allen
 
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#4
Those injectors share a fuel rail, and I believe the fuel line connects to it. Maybe those injectors are more plugged that the other bank, causing a greater tendency to misfire. A cheap and easy test would be to swap injectors from side to side. If you have any leaks, you will have to replace o-rings.
 

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Danica, thanks for that suggestion. I guess cheap is relative. On the last injector I replaced on a Dodge, I was told that it didn't come with the o-rings. The two o-rings from Dodge cost me over $9! Fortunately, the parts guy was wrong - the injector did come with o-rings. The lesson I learned was that o-rings should be sourced through the aftermarket - even if I will only use OEM parts for sensors, injectors, etc.
 

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Compression testing has lost favor for some years but is making a big comeback as a diagnostic tool. Many places can do running compression tests now. The important information it can give you is cylinders that are equal or one low that can cause misfires under certain conditions. I highly recommend it as a tool. We had a Focus in the shop on Saturday with an occasional misfire, oil inside the plenum & air snorkel. Compression test showed cyl 2 at 65 psi, the other 3 were 170 psi....why was it missing only sometimes??? The bottom line is the test shows if the engine is basically sound or not. Transman
 

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Transman, I'm not familiar with a running compression test. I assume that there is special equipment involved. You've piqued my curiosity. Please tell me more.
 

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It is VERY time consuming, you use a compression tester without the schrader valve in the tube. Start the engine and see what the compression is, then do the next cylinder. The compression ratings will not be as high overall but very accurate. By the time you get to the last cylinder it tends to get fairly warm. Transman
Here is a good explanation from the web:
This is a summary of the responses to a question about a "Dynamic Compression Test" sent out via the i-ATN e-mail list and posted on Compuserve's "For Techs Only" forum. It seemed to ring a bell with the most techs as a "running compression test," so I will use that name here. Call it what you will, this test is an accurate if slightly esoteric and time-consuming test of cylinder breathing. It is in fact recommended by Detroit Diesel instead of a traditional static compression test, it is included as part of Delmar's ATTP program, and several instructors use it as part of their state emission training programs.

HOW TO PERFORM A RUNNING COMPRESSION TEST

1. Start with a normal ("static") compression test. To eliminate rings, valves, holes in pistons, that sort of things. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm.

2. Put all spark plugs but one back in. Ground that plug wire to prevent module damage. Disconnect that injector on a port fuel system.

3. Put your compression tester into the empty hole. The test can be done without a Shrader valve, but most people recommended leaving the valve in the gauge and "burping" the gauge every 5-6 "puffs".

4. Start the engine and take a reading. Write it down

5. Now goose the throttle for a "snap acceleration" reading. Reading should rise. Write it down NOTE: Don't use the gas pedal for this snap acceleration. The idea is to manually open then close throttle as fast as possible while without speeding up the engine. This forces the engine to take a "gulp" of air.

6. Now write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe 2-3 good ones. Make a chart like this: CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 125 Cyl 2 175 80 130 Cyl 3 160 75 120 Cyl 4 160 80 125

7. ANALYSIS: Running compression at idle should be 50-75 psi (about half cranking compression). Snap throttle compression should be about 80% of cranking compression.

EXAMPLE 1 - RESTRICTED INTAKE CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 80 If Snap reading is low (much less than 80% cranking compression), look for restricted intake air- severely carboned intake valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the runners. (Toyota, Vortec etc. with variable runner length) Comparing measurements between cylinders is important.

EXAMPLE 2 - RESTRICTED EXHAUST CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 180 If snap measurements are significantly higher than 80% of cranking measurements, look for restricted exhaust on that cylinder-such as worn exhaust cam lobe, or collapsed lifter. Or, if they are all high, look for a clogged cat converter.

WHAT IS GOING ON?

When you do a normal compression test, you are checking cylinder sealing, not cylinder breathing. When you check engine vacuum at the manifold, you are looking at the breathing of the entire engine, by checking vacuum at a common (plenum) source. You aren't testing a specific cylinder. This test looks at the breathing of an individual cylinder.

Say the engine is running at 18 inches vacuum. Atmospheric pressure is about 30 inches, so the difference (30 inches - 18 inches = 12 inches) is what the engine is sucking in. 12 inches mercury is equivalent to about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you should get 6 x 8 = 48 psi pressure if all the air makes it into the cylinder and then gets pushed out. So your idle reading on running compression is about 50 psi.

When you snap the throttle, the manifold vacuum drops, so the absolute air pressure going into the cylinder increases.

In fact, you can do running compression tests at various constant manifold vacuum readings (by brake-torqueing the engine momentarily), and the running compression should roughly correspond to the manifold vacuum. For example, at 10 inches vacuum, engine should be breathing in about 10 psi air pressure, so you should see a running compression reading of about 80 psi (at 8 to 1 compression ratio).

If one cylinder reads low running compression compared to the rest it means that the air didn't make it in. If one cylinder reads high, the air didn't make it out (and the next pulse of air raised the pressure).

Many thanks to the people who responded through both Compuserve and i-ATN. Special thanks go to Bruce Amacker, Bob Cammarano, Chris Chesney, Troy Croskey, Don Hazlett, Rick Jensen, Greg McConiga, David Palin, & Thomas ??(whose last name I seem to have lost)
 

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Transman, thank you very much. Very interesting info. Once again...you're the (trans)man! :ROFL

Next time I have the truck and if i have some time to kill, I'll try that out. Thanks again...Allen
 

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Obviously, this is not something you do on a normal tune up but if you are having one kick your butt around and NEED to know the engine is sound, this is the test. Good luck, Transman
 

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#11
Transman, I've been meaning to comment on what I assume is a quote under your name - I think that is great. Can you tell me to whom it should be attributed? I'd like to make a sign for my shop with that quote.
 

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LOL, It's all mine baby. I keep it on the front of my brain to keep me from taking myself too seriously and keep me humble. The knowledge I have acquired over the years has come from books, teachers, other peoples mistakes and even on occasion... my own mistakes (rarely of course). Feel free to use it if you wish, Transman