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Dodgerench, ASE Certified Technician
Category: Dodge
Satisfied Customers: 3404
Experience:  30+ years Dodge/Chrysler exp., ASE Master with L1 certification. Driveability/ combustion specialist
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I have a 98 8.0L V10 Dodge Ram. When it is cold, it runs very

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I have a 98 8.0L V10 Dodge Ram. When it is cold, it runs very rough around 1700 RPM (about 55 in 5th gear). Also, even when warmed up, it runs very rough when I am in the same range but in low gears (on a work site I have to maintain 25mph and that seems to be the worst for the engine). Above or below normal running speeds it smooths out. Any help is appreciated.
HiCustomer welcome to Just Answer!

It sounds more like it might be a rotating imbalance, not necessarily an engine problem. Imbalances like this have a tendency to find a sympathetic RPM that transmits its vibration frequency to the engine and to the body, like the 1700 engine speed you noted. Above or below this, there will be counteracting vibrations that somewhat cancel it out.

Take a look at the water pump and radiator fan for looseness or bent blades. While clutch fans don't usually get involved in imbalance problems like what you've described, it sure could if the water pump shaft is wobbling enough.

Other places to look would be the front pulley at the crankshaft, where you may have picked up a load of mud or something at the job site. They'll tend to sink to one side and harden, leaving a pretty considerable imbalance at the front of the engine.

If the torque converter or clutch has been replaced just before this started, that would be another possible source of imbalance.

If you feel that it's engine misfire or something related to engine combustion, let me know a little more about the way it runs otherwise. You won't have a Misfire Monitor on your V10 since it's not a full OBD2 certified engine, but there may be codes stored in the engine that could be useful. Unfortunately, there's no way to extract codes from the 98 model year without an actual scan, something that actually can be done free of charge at your local AutoZone auto parts store.

Check out those few things and let me know if we need to keep looking.

Talk in a bit,
Customer: replied 7 years ago.
Interesting you should mention the clutch since it is slipping and needing to be replaced.
However, I'm pretty sure it's more about misfire. I actually drove it into town (mostly do country driving) and didn't realize how bad it really was. It's now backfiring and lurching with loss of power from stoplights until I really give it the gas.
When I first got the truck and started driving it, I did get an engine light. I own an OBD2 but didn't realize disconnecting the battery would clear the codes. The previous owner put a trailer brake control right under the connector and to remove it, I pulled the battery cables. When I hooked up, there were no codes and the check engine light cleared. I haven't gotten one since.
After tonight's drive, I'm thinking it would be easier to explain the times it runs well, than when it runs rough. I drove back from town (about 20 minutes) and by the time I got back, it was running pretty decent again. It seems like stop and go, cold and slow driving really make it rough.
To describe the "roughness" when it's really bad: When the light turns green, I have it in 2nd gear (1st is like granny gear); the truck sort of sputters and kicks a little, sometimes makes popping sounds and backfires; if I keep pushing the accelerator down, it kind of goes quiet and loses power until I get 75% down, when it suddenly kicks in and I get full power with normal engine sound and no backfire; on wet pavement this will cause the wheels to slip it takes off so fast; then I can shift to 3rd and go through a little of the same thing again. If I have to stop soon after that, it does this again. If I can drive a few miles, it won't be so bad at the next light. Does that make any sense?
Thanks for your help.
Thanks, Matt. I wasn't sure what we had because you mostly mentioned roughness but I see we've got quite a bit more than that going on.

It sounds like a bad oxygen sensor from what you've told me. 02 sensors are used to fine-tune your air-fuel mixture and should keep things pretty close to the engineering ideal of 14.7:1 that you've heard of when they work right. Their varies with exhaust stream oxygen content, something that can be used directly by the PCM so it knows which way to kick the mixture.

To work properly, they need to be heated and heated well so they're equipped with electrical heating elements that operate continuously as the engine is running. Your PCM knows better than to look for their signal before enough time has elapsed for heating, a time called open-loop operation.

Once open loop is timed out, the PCM goes closed-loop, where it actively solicits the sensors' signals for mixture control. If the sensor is healthy AND heated... great. If not, you can have some pretty nasty driveability.

Part of the problem lies in the way oxygen sensors are checked using the OBD process. The PCM places a very weak 5v bias voltage on the sensor signal circuit, something that's easily consumed by the sensor if it comes online properly... it just melts away.

But if the sensor doesn't reach operating temperature, part of the bias voltage sticks around and closely resembles that of a rich air-fuel mixture which prompts the PCM to take fuel away until the engine barely runs.

Since heat is what it craves, you've noticed that slow speed in-town or cold weather driving tends to affect the truck worse than being out on the open road. Heat is heat and you do satisfy a certain amount of 02 sensor heating with the exhaust stream, which will be hotter with increased engine load.

You also found that pushing the throttle eventually pays off and the engine responds even when it's running at its worst. At a throttle angle that's close to wide open, your fuel system reverts to open-loop, a time when oxygen sensor output isn't used. This also takes you well past the corrupted fuel cells that had been corrected to a value of up to -70% back to one that's zero... even. When that one hits, the wheels spin!

It makes sense. Unfortunately I don't know which sensor is messing with you at this point and it could even be both sides (right and left banks) if the heating circuit isn't powered up. There will be a dedicated fuse underhood marked 02 SENSOR, which provides the output power source to your sensor heaters. It might be worth checking to make sure it's not popped. This fuse is powered only when the engine is running by the way.

Check it out and let me know what you find, Matt. I'm sure the offending sensor will eventually code again, but if they're both the same age it might be wise just to stuff a couple fresh ones in.

Codes I'd expect to see would be 02 SENSOR HEATER CIRCUIT, 02 SENSOR SHORTED TO VOLTAGE or possibly FUEL SYSTEM RICH. Each code would come with a designation for side-to-side, but that's the gist of the matter...

Talk in a bit,
Dodgerench and 3 other Dodge Specialists are ready to help you
Customer: replied 7 years ago.
Thanks. I'll focus on those.
The main reason I haven't just replaced them wholesale (apart from the $400) is that I have dual exhaust and the Dodge dealer tells me there are 4 of them; 2 before the cats and 2 after, and they are different models (although NAPA sells the same one for all positions). After I check the fuse, is there a set you would go after first?
Thanks again for the help.
I don't think you'll have downstream sensors, Matt... unless your truck is a special emission market like CA, NY or MA. As a matter of fact, if you've got an air pump, I'm 99% sure that you will have only upstream units.

Those would be the ones to concentrate on anyway. Upstream oxygen sensors are the ones used for mixture control... downstream is mostly used for catalyst efficiency monitoring and has very little influence on fuel trim. To tell for sure, look for an oxygen sensor on each exhaust system just past the cats... there isn't much room to put one in there.

One thing that occurred to me late last night was that you could probably unplug each bank separately to see if it makes a difference, since an open circuit will push 02 signal voltage to 5.0v, making it easy for the PCM to see a failure in the particular 02 circuit and disregard its signal. This should ensure open loop operation continuously on whichever bank you choose, so the engine should run fine... or at least better!

One word of caution. I've seen model years where the factory placed both 02 sensor connectors within inches of one another in the harness which means they can be swapped side-to-side easily if both are disconnected at the same time. Swapping sides with 02 sensors means the PCM will be monitoring the opposite engine bank for each of its fuel management systems, usually resulting in a problem similar to what you're fighting now... the adaptives are maxxed out, either rich or lean and sometimes one of each. Since the truck is fairly new to you, inspect the harness carefully for R or L designations on a tape band by the harness connector just in case the previous owner got 'em mixed up. We can always use wire color to be sure.

Something tells me that you won't find the 02 sensor fuse blown, but it's worth checking out just in case. When 02 sensors fail to get any heating at all is when you'll get fast responses from the on-board diagnostics... 5.0 volts on a zero to 1.0 volt circuit shows up pretty big on the radar at that point.

A battery disconnect before disconnect-type testing or after sensor(s) are replaced will clear all adaptives, putting the fuel system back at its original engineering ideal of zero trim either way and will ensure there won't be any odd holes in the powerband as adaptives reset themselves.

Many thanks, Matt. Be sure to write back when you've got a diagnosis!

Customer: replied 7 years ago.
Dear Ed,
I thought I would give you an update of my progress.
As you recommended, I went out to investigate the heater fuse. I found the fuse pretty easily, pulled it out, and it wasn't blown. It seemed a little corroded on the edges so I checked it with the voltmeter anyway. There were places that the corrosion prevented continuity. When I looked into the socket, one of the legs was pretty corroded. I got some contact cleaner and did a little bit of cleanup, but really didn't think it was going to do anything.
Then I got in to start the truck. It wouldn't start and acted dead. When I checked the battery, it was down to 8V. This caused quite a distraction because I thought I had done something. I disconnected the battery and removed it to see if I had disturbed some wiring. After finding nothing, I put it back in, charged it up, and the truck started fine. Since I didn't find anything, I didn't drive it for several days for lack of trust. I started driving it Wednesday again.
Now it runs the same hot or cold and very smooth with lots of power. I know you mentioned the adaptives will reset with the battery disconnect, so it's hard for me to tell which one made the change, but I'd like to believe it was the heater fuse. I guess I'll know as a few weeks pass and the adaptives begin again.
Either way, I now have enough power that the clutch is a serious hindrance. I'm going to have to have that addressed soon so I'll be dropping it off at the mechanic's. Maybe I'll have enough info by then to determine if I should go ahead with the O2 sensors or maybe I'll do it anyway while it's there.
Thanks again for the advice. While in there I noticed there is a corrosion problem I need to work on, not just in that fuse socket, but in other locations including the main power connection to that box from the alternator. I bought the truck from a fellow in Vermont; they must use a lot more salt on the roads than we do in northern Ohio!
Many thanks for the update, Matt! I hadn't considered a reduced power flow to the sensors... only good flow or NO flow. It sure makes sense that a high resistance point in the circuit would delay sensor heating just like a bad sensor, so you may have located and fixed the problem on your own.

I sure appreciate that you took the time to fill me in on something I'll consider from this point on when it comes to problems like you had!