1999 Dodge Ram

HEDINE

MEMBER
1999 DODGE RAM
V8
4WD
AUTOMATIC
133,000 MILES

When I turn my truck on, it fires up and then only idles back down to about 1400 RPM (usually around 8 or 900). I can cruise on flat ground at 35mph and take my foot off the gas, the truck will hold that speed until I brake because of the high running speed. The CEL came on at the same time that the problem started. I found a vacuum leak and fixed it, have replaced the idle air control valve, the MAP sensor, and have cleaned the throttle body. The error code says replace the idle air control valve, which I have already replaced even though the symptoms didn't suggest it (I have replaced IACV twice before). I have followed the vacuum hose as far as I can but haven't found any cracks. When this problem first started, when cold, the engine would idle fine but as it warmed up it would begin to idle faster. Now, it idles high as soon as I start it. If I coast down from about 45 or 50, a few seconds after I take my foot off the pedal I can feel and hear it drop down to normal speed momentarily, then speed back up. I have a K&N intake, shorty headers, and an aftermarket muffler with no cat, if that means anything.

5 Replies

CARADIODOC

MECHANIC
33,910 POSTS

The modifications add a bunch of variables. Are there still oxygen sensors in the exhaust system? Without catalytic converters, the engine computer is going to turn the Check Engine light on and set a diagnostic fault code related to converter efficiency.

High, varying idle is a sign of a vacuum leak. Once you repaired the leak, did you erase the codes? If not, the codes will remain until the engine is started around 50 times without the problem coming back.

Caradiodoc

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Thursday, March 11th, 2010 AT 5:32 PM

HEDINE

MEMBER
3 POSTS

The cat has been out since last summer, the O2 sensors are still there, I used a spark plug spacer to back the downstream sensor out of the pipe, everything has worked no problem since then, no CEL ever came on. The light came on right when the high idling began. There is no error code relating to the O2 sensor or cat, only the IACV. The K&N has been on for about two and a half years and has never caused problems. I have not had the codes cleared, but its not the CEL I am worried about, it's the symptoms alone. The thing thats throwing me off is that when I coast, the run speed drops drastically, suddenly down to normal for less than a second and then back up. It is not simply fast idle speed. On flat ground from a full stop, I can accelerate up to just under 30mph without touching the gas, and maintain 40-45mph without pushing the gas pedal for miles. And I can not find another vacuum leak anywhere, the hose is in good shape and all the rubber connectors are good.

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Friday, March 12th, 2010 AT 6:53 PM

CARADIODOC

MECHANIC
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You're right, I don't think it's related, but the front O2 sensor switches rich - lean very rapidly. When the catalytic converter is working properly the second sensor will switch very slowly. As the converter's efficiency goes down, the second sensor's switching speed approaches that of the first sensor. That's how the computer knows nothing is happening in the converter. My guess is by using a spacer with the second sensor, it isn't able to see the unburned oxygen so it responds slowly. How did you figure out to do that?

If you're confident there are no vacum leaks, the next step would be to connect a scanner to read the Automatic Idle Speed motor "steps". Typical for a properly running engine at idle is around step 32. If you find a higher number, say around 50 to 80, the Engine Computer is trying to increase engine speed. More commonly you will find it near step 0. That indicates the computer sees the engine speed is too high and is trying to bring it down.

You can also read the O2 sensor voltage while driving. If it mostly stays in a "lean" condition, that would still point to another vacuum leak. If you can catch the high idle while you're under the hood and the engine is still cold, spray water from a squirt bottle onto suspected hoses and gaskets and watch for a change. One relatively uncommon thing no one thinks to look at is the bushings for the throttle blade. A leak there should be met with the computer closing the Idle Speed Motor to a lower step.

Caradiodoc

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Friday, March 12th, 2010 AT 9:41 PM

HEDINE

MEMBER
3 POSTS

My apologies, school and work have been hectic, but I was able to make an advancement.

First, in answer to your question about the O2 sensor, I just looked at online forums about removing catalytic converters, that was the advice given. Backing the sensor out of the pipe also lowers the temerature of the sensor which, from what I have been told, makes a difference.

Now on to the truck. The error code continually came up with an IACV message even after I replaced it twice so I busted out the multimeter and found a wire with spotty continuity. I cut it at both plugs and replaced the wire, some symptoms subsided. In an earlier post I said something about feeling the engine momentarily slowing down when I coasted, then speeding back up quickly. That has stopped, and the speed is less varyable now but still too high. I took it to a mechanic to have the IACV adjusted, he brought the engine speed as low as he could but it was still far from right.

When I turn on my truck cold, it runs barely above normal, about 1000 RPM. When I drive into town, if I stop at a stoplight or something my engine runs at normal speed, about 800, but uneven. If I shift into park, it only goes up to about 1100 and runs even. If I turn my truck off and turn it back on while it is still warm, whether its ten seconds or over an hour, the RPMs are at 14-1500. This makes no sense to me.

The diagnostics now show a fuel mixture problem, I believe it was lean but it has been a few weeks since I was able to get it in. I cleaned my O2 sensors with no change. I used my vacuum diagram and found the hose for engine speed control, I removed the hose from the manifold while running and the speed immediately went up, I put a finger over the manifold hole and the end of the hose, the speed came back down to where it was and both fingers had strong suction on them, could that rule out a vacuum leak? If not, is there another way I can? I followed the hose all the way to the end where the hose becomes a metal tube, no cracks anywhere.

I usually get about 13 MPG city, I am currently getting about 5.5 and three different local shops have had no luck. Thanks for any help you can offer.

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Wednesday, April 14th, 2010 AT 3:11 PM

CARADIODOC

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Try pinching off all of the vacuum hoses at once in case there is a leak in more than one.

If the computer does indeed think the exhaust is too lean, it will lengthen the on-time for the injectors resulting in too much fuel and poor mileage. With a vacuum leak, the computer should be able to achieve a proper mixture just by adding fuel, but it can only adjust approximately plus or minus about 10 percent. But, with an air leak in the exhaust system, the O2 sensor will always see the unburned oxygen regardless of how much extra fuel it adds. You should see black smoke from the tail pipe even though the computer sees a lean mixture.

I've never tried this before, but you might get a clue by driving with the two-wire coolant temperature sensor disconnected. The Engine Computer won't know when the engine is warmed up so it won't know when to go into closed loop which simply means it starts making fine tuning adjustments to the mixture based on oxygen sensor readings. The Check Engine light will turn on when the coolant temperature sensor problem is detected, but if your fuel mileage improves, that would prove false readings from the O2 sensor are the reason for the poor mileage. Unplugging the front O2 sensor instead of the coolant temperature sensor might work too.

Caadiodoc