# RPM Surge?



## xxxxxxBirdDogger (Mar 7, 2008)

2000 Dodge Durango- The engine runs smoothly at low speeds and everything seems fine. At about 70 mph, the RPM's surge and the engine gets a case of the hiccups. The vehicle hitches repeatedly at that point and then the check engine light comes on. Then after the vehicle has been turned off the check engine light turns off and everything is smooth again until the next time it gets taken up to 70 mph. 

Any idea what the problem might be?


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## stimmie78 (Dec 8, 2007)

Someone installed a speeding alert system. Since the freeway speed limit is 65 the truck is letting you know to slow down and obey the law. It was all part of the bailout cash from Uncle Obama...


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## xxxxxxBirdDogger (Mar 7, 2008)

> Since the freeway speed limit is 65 the truck is letting you know to slow down and obey the law.


I know that was a joke, but just FYI- the speed limit is 75 where I live. :mrgreen:


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## Huge29 (Sep 17, 2007)

I am really confused how the speed would have anything to do with it; at the same RPM, but in 3rd gear the engine is doing the exact same thing. That is very odd.


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## nickpan (May 6, 2008)

Don't know much about the durangos but in some other vehicles i would be looking towards the cam position sensor or a shaft speed sensor that relates the speed of the motor to the actual speed of the driveline. Again, i have no knowldedge of durangos. Search on the internet, alot of auto repair forums out there


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## jahan (Sep 7, 2007)

It is because it is a Dodge, sorry couldn't resist. I agree with Huge, that is weird that it only does it at 70, you will go past that same rpm several times on your way up to 70 I would think. Have you noticed the exact rpm that it happens at? It has got to be some kind of a sensor that is triggering it, does it act like it down shifted at all, maybe the the transmission is trying to downshift for whatever reason. I am assuming it is an automatic.


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## luv2fsh&hnt (Sep 22, 2007)

My '01 Ram surges at idle.I haven't noticed it while traveling. It did have issues shifting when I had the cruise set.Once it downshifted when climbing a hill it wouldn't shift after cresting the hill. I had the tranny flushed and the filter changed and it didn't happen anymore. It could something as simple as the throttle body being dirty and needing a good cleaning.


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## Spry Yellowdog (Sep 8, 2007)

I would start by finding out what code is being set when the check engine light 
comes on.
Even though the light goes out a code should be stored in memory.

Spry


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## .45 (Sep 21, 2007)

Spry Yellowdog said:


> I would start by finding out what code is being set when the check engine light
> comes on.
> Even though the light goes out a code should be stored in memory.
> 
> Spry


Good idea.....when the engine is surging it is usually looking for the right mixture of fuel and air. 70 mph might just be all the wrong conditions for the fuel/air mix to work properly.

My guess would be a vacuum leak. Hoses, intake manifold or throttle body.


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## xxxxxxBirdDogger (Mar 7, 2008)

> I would start by finding out what code is being set when the check engine light
> comes on.
> Even though the light goes out a code should be stored in memory.
> 
> Spry


Hey, thanks. I didn't know that the code would be stored. That's some good information.

This definitely feels fuel related to me. I've replaced two transmissions in other vehicles and it's not quite like that. There is absolutely no RPM surging at lower speeds. It runs like a champ below 70 mph.


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## Huge29 (Sep 17, 2007)

Is the check engine light coming on? If so, it does store the code. Oreillys and autozone will both lend the OBDII diagnostic tool to find out the code.


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## xxxxxxBirdDogger (Mar 7, 2008)

> Is the check engine light coming on? If so, it does store the code. Oreillys and autozone will both lend the OBDII diagnostic tool to find out the code.


Yes, the check engine light comes on when the surging happens. Then the light is off again each time I restart the engine.


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## Huge29 (Sep 17, 2007)

That should still be readable; go to AZ or Oreillys to get it. They can retrieve the code or just drive right to one from the freeway while the error is still showing. The codes are a little vague, but give the general issue. This one really has me curious. Got to be something like a cam sensor or in that computer sensor realm...


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## cowmilker (Dec 17, 2008)

Could be o2 sensor, or Throttle Position Sensor, or MAP (or cam or crank like everyone else has said). Could be that when you are cruising at that specific throttle position (like say 1/4 throttle) the TPS is in it's "worn spot" but when you are accelerating at that specific RPM your throttle is open more. Could be that when you are cruising at highway speeds there is not enough exhaust gasses going past the 02 sensor to keep it hot (they have to be warm to work good, that is why they are located closer to the engine, some of them are even heated), but as you are accelerating there is enough "hot" exhaust to keep it warm (plus at wide open throttle some cars don't take readings from the o2 sensor). MAP sensors take vacuum readings to feed the computer, if it happens to be worn (or there is a vacuum leak) it will not read constant signal.

For what it's worth I'm dealing with a similar problem on my 92 f-350 (wide open throttle is usually good but just "cruising" it lurches and farts and pops) my next sensor to try is the MAP.


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## xxxxxxBirdDogger (Mar 7, 2008)

OBD Diagnosis: Fuel injector #7 firing improperly.

Options to fix this problem: Replace fuel injector #7 or the fuel injector driver circuit. 

I changed the fuel injector first because it's about $350 cheaper than the driver circuit. Cleaned the throttle body as well. Crossing my fingers now.


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