Slight hessitation when accelerating to highway speeds

[Wander]

97 DI-4.0 5 speed. I've had a slight, short hessitation when I am getting up to highway speeds so during continuous acceleration. It only lasts a second or two and feels like it is staving for fuel or spark. Only occures during longer acceleration, never around town or any other time. Is this a failing coil pack? I do know the fuel pump and filter where replaced shortly before I got her so I don't think that's it. No issues with hard starting so I don't think it's the injectors but I just ran some seafoam through in case-no change. Overal it runs like new and this is the only issue I have. Any ideas?

 

[Wander]

To add to the info-from the research I've done it looks like it may be the throttle position sensor (TPS). Anyone had this similar problem and did a new sensor fix it?

 

[cosmic88]

clear confusion

Ahh the intricacies or Rovers... God Save The Queen!

Easy test of the TPS. Grab your Multimeter. In this instance I prefer a digital meter but an analogue meter will do just fine.... No need to remove the TPS itself... just its plug.

Separate the TPS plug from the Harness connection. Find the two pins on the TPS plug that give you a resistance value and that value will change as you open the throttle... on my TPS there are three wires; blue, green and brown. With the Brn and grn wires I know the value changes from 6.4k ohms with closed throttle down to 1.85k ohms at wide open for the early Lucas EFI's. Whatever the base resistance value is for GEMS, during the slow and very steady opening of the throttle you should see a smooth and seamless change in values (ohms or volts). IF there is any gap or rapid loss of signal then the TPS is worn internally and no good anymore. The voltage throughout that full range of motion for GEMS EFI's is 0.6 at closed throttle and up to 4.5 volts with the throttle wide open.

Sorry I don't have the exact Ohm value for your MY but I'm an old skool 14cux fan and I'm keeping it that way. :ylsmoke:

I know you say the pump and filter were changed but have you checked the pressure at the rail? Could also be a sign of a failing regulator. Could also be an intermittant VSS, But if it were the vss going bad then you would lose injector signal to half of the engine. THAT would be a big drop off in power.

 

[Dendy Jarrett]

You should also check the stepper motor just to make sure it is clean. This usually affects idle, but can cause hiccup on acceleration.
(if I remember correctly).

D

 

[Wander]

It acted up this morning but this time it was early in and around 30 mph. Since I was going slow I was able to pay more attention and watched the tach as I feathered the gas to get it past the spot. As I moved the throttle in and out the tach did not move at all and neither did the engine revs rise or fall so now I'm thinking it must be the TPS. I am planning to replace the TPS and the VSS to be overly cautious and go from there.

 

[cosmic88]

Sounds like a good plan... And just for fun (elimination of possibilities) Fuel pressure check is free

If you opened the throttle and had no increase in revs my money is on TPS. Let us know the results.

 

[rover4x4]

Hows the alternator maybe suspect since you mention the tachometer acting up.

 

[rijosho]

x2 alt if the actual gauge for the tach was not registering. Common symptoms of a shot alternator.

 

[Wander]

Good idea on the alt- I had trouble starting it today until I flipped the fan off, it's hot so I thought it might be the heat but a failing alt could also explain it. It's so new to me I'm still learning it but I do remember that a dead tach was the first symptom of a failed tach on a 97 XD at this years URE Safari. My tach hasn't failed and I only noticed it not registering when it hessitated and figured if the sensor is going then it wouldn't send the signal to the computer/tach either but I could be wrong that they are connected.

 

[sven]

I always start with the simple stuff first. How are the plugs and wires? If in doubt, replace them.

Just replacing the TPS on a GEMS truck can cause other problems like making your base idle all out of whack. Then you're gonna need a testbook to readjust your base idle. Try testing the TPS using a multimeter like somelse pointed out earlier...dont just replace it on a guess.