99 Dakota engine doesn't hold steady speed

[Pop_Rivit]

I have a neighbor with a 1999 Dakota with (I think) the 5.2 V8 engine. The pickup seems to run great with the exception of one issue that started this summer.

If you're driving on a flat highway and holding it at a steady speed (such as 55 mph) every minute or so the engine seems to stumble a bit-there is a change in engine RPM's of about 100 RPM's-sort of a bounce up and down. It feels like a misfire, but it's so infrequent that I'm hesitant to call it a misfire. It only lasts a second, and then the engine is steady for another minute or so.

It doesn't happen if you're on hills or varying the speed with the throttle, only when the engine is held at a steady highway speed. It also doesn't happen all the time-sometimes he says that it doesn't happen for a day or two, then it will start again.

There is no check engine light and there are no stored codes. Earlier this year he did have a check engine light and the code was for the #7 fuel injector. He ran a couple bottles of fuel system cleaner through it and the check engine light/code never returned.

Any suggestions on what might cause the stumble on occasion?

Thanks!

 

[Boomer]

Could be a vacuum leak. Check around all the hoses that might be involved. Is this point injected or a throttle body type engine? (Don't know Chrysler products.) If the latter, check for bolt tightness around the base of the throttle body. It may also need to have the fuel filter replaced.

 

[Jimmy9190]

Check the throttle position sensor. It's on the left side of the throttle body. It should have 5 volts across the 2 outside wires on the 3-pin wire connector plug. On the center wire it should have about a quarter to half a volt with the ignition on, throttle plates closed and the engine not running, and the voltage should slowly increase as you open the throttle by hand. The center wire should have about 4.5 volts on it with the throttle wide open.

It's going to be hard to diagnose though since the problem is so intermittent.

 

[motorguy222]

Does it use any oil or smoke at all?

 

[Scienceman]

I would change the fuel filter.

 

[Stu_Rock]

Originally Posted By: Jimmy9190
Check the throttle position sensor.

I like this idea. It could be a dead spot in the potentiometer in the TPS.

In light of the injector issue, I also would be interested to watch on a scan tool how the air/fuel mixture is being adjusted during one of these drives.

 

[Jimmy9190]

The fuel filter is an integral part of the fuel pump assembly. The truck does not have any type of inline fuel filter. A new TPS will cost around $50.00. If the output from the original TPS is out of normal range, it would be best to replace it anyway.

I am no Dodge mechanic but to me this sounds like a bad TPS because it only happens when the truck is at a steady speed and RPM.

Just my 2 cents.

 

[CBR.worm]

A/C compressor cycling?

 

[Pop_Rivit]

Originally Posted By: Boomer
Could be a vacuum leak. Check around all the hoses that might be involved. Is this point injected or a throttle body type engine? (Don't know Chrysler products.) If the latter, check for bolt tightness around the base of the throttle body. It may also need to have the fuel filter replaced.


I'll take another look for vacuum leaks.

Originally Posted By: Jimmy9190
Check the throttle position sensor. It's on the left side of the throttle body. It should have 5 volts across the 2 outside wires on the 3-pin wire connector plug. On the center wire it should have about a quarter to half a volt with the ignition on, throttle plates closed and the engine not running, and the voltage should slowly increase as you open the throttle by hand. The center wire should have about 4.5 volts on it with the throttle wide open.

It's going to be hard to diagnose though since the problem is so intermittent.


After hearing the description it sounds like this is a possibility. Since they're only $50 or so I might change it out anyway to make sure.

Originally Posted By: motorguy222
Does it use any oil or smoke at all?


No, none at all. He's been good about maintenance and has owned it since new.

Originally Posted By: CBR.worm
A/C compressor cycling?


It sort of feels like that, but much more significant. I turned the AC on while testing it and it made no difference. I'm pretty sure the AC is always off unless it's intentionally turned on.

One thing I didn't do was test it at differnent speeds. I might try a 70 MPH interstate drive to see if it happens at speeds higher than 55 MPH. I might also try a 35 MPH steady drive and see if it happens at lower speeds too.

If I understand it correctly, if it doesn't happen at lower or higher speeds that would also point to the throttle position sensor as the culprit, correct?

Thanks for the help!

 

[rslifkin]

It's most likely the throttle position sensor. It can develop a dead spot over time on these engines, then it will cause the transmission to bounce in and out of TC lockup when cruising at a steady speed, causing the bounce in rpms.

 

[mechtech2]

Don't forget the basics.
A tedious cleaning of your power and ground cables [and where they seat] should be performed.
Bad grounds, or the ground/power to the computer cause all sorts of odd problems.
Hundred of dollars are spent looking for problems that are simple to fix.
You only gain by doing this. If it does not fix this problem, it helps everything work right anyway. Good maintenance.

 

[Hokiefyd]

Does his 5.2L have EGR? Some did and some did not. If it does have EGR, it's just another system to check...make sure the EGR valve moves freely, has a good source of vacuum, etc.

 

[VNTS]

Classic TPS, get a new mopar not a aftermarker.

 

[rslifkin]

A 99 5.2 doesn't have EGR.

 

[Pop_Rivit]

Turns out the throttle position sensor was the cause of the problem. I was surprised at how easy it was to replace-it took longer to find my T20 bit than it did to actually replace it. We replaced it on Wednesday and he has not had a problem since.

Both John and me thank everyone for the suggestions and advice.

 

[Jimmy9190]

Good. Glad to know we could help and it was an easy fix.