misfire ?

[Jeff B]

Quick question- 2 months ago my D90 wouldn’t idle at stops. After some quick internet diagnosis, I swapped out this thing for another old one I had sitting around. Miraculously, the idle stall was cured! But, last week it started missing while accelerating. It will still idle fine. (Plugs and plug wires have less than 3000 miles on them)
1997 D90 4.0

 

[Jeff B]

This is the part. Throttle sensor?

 

[rocky]

Check spark plug condition, color and gap
Check plug wires properly seated
Distributor cap and rotor condition and age?

 

[Uncle Douglas]

Since there is no distributor on a 4.0 it makes things much simpler diagnose. I have a Bluetooth/ iphone obd2 scanner if you want to come over to my side of the mountain.
First thing I'd try is run the engine @ night in total darkeness and see if you can discern any arcing.
Just because something is new doesn't mean you can rule it out- these are rovers...
Pull the plugs and see if one is fouled. Coil packs do fail from time to time.

Yeah,thats the TPS.

 

[rocky]

My bad...UD.... Would have thought an OBD2 would code if there is a misfire

 

[jymmiejamz]

This is the part. Throttle sensor?

That is the throttle position sensor.

 

[BarryO]

Check that there's no air leaks in the sealing/gasket for the new TPS. If there are, this can cause misfires in the 4.0 engine (I spent a chunk of change chasing something similar down).

 

[Jeff B]

Thanks guys--- let me clarify; I did get a code- 1316?
Didn't tell me much. I just have the ScanGauge2.

I will definitely do the darkness/sparks check UD.

And I'll check that I put the used throttle position sensor in with no leaks since that is the only thing that has changed lately.


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[Uncle Douglas]

p1316 just tells you what you already know, its missing and has excessive emissions as a result.
You can usually hear a spark short/jump to ground

 

[RBBailey]

Check that there's no air leaks in the sealing/gasket for the new TPS. If there are, this can cause misfires in the 4.0 engine (I spent a chunk of change chasing something similar down).

I wondered what came of your problem. Was it causing misfire (the thing was SUPER quiet and smooth) or was it just causing a code to flash?

 

[BarryO]

I wondered what came of your problem. Was it causing misfire (the thing was SUPER quiet and smooth) or was it just causing a code to flash?

It turned out to be a pinhole leak in the intake plenum. Caused enough of a variation in crankshaft speed that the ECU picked it up as a misfire.

 

[jymmiejamz]

Check that there's no air leaks in the sealing/gasket for the new TPS. If there are, this can cause misfires in the 4.0 engine (I spent a chunk of change chasing something similar down).

I may be wrong, but I don't think there is a gasket for the TPS in a gems truck. I think there is only the seal for the throttle shaft.

 

[BarryO]

I may be wrong, but I don't think there is a gasket for the TPS in a gems truck. I think there is only the seal for the throttle shaft.

OK I wasn't sure; just thought I'd throw that out, since that's what he changed. These engines seem to be sensitive to air leaks past the MAF

 

[Jeff B]

- update July 5... I didn't do much. Cleaned up the original Throttle sensor(made sure the small o-ring was there) checked for obvious air leaks. Did not find any(I might get motivated enough to buy a fancy silicone hose to replace my original air cleaner-to-plenum hose) but it was ok.

Current status: On start up it revs @ 2k for several minutes, but after driving a few blocks, it's back to normal---idling around 800rpm at stoplights and such. No misfires at all today; smooth as butter.
Just the really high start up idle?!?
No codes, etc.

 

[Uncle Douglas]

Green 90 has done that. With the HD fan clutch it makes a ton of noise.
Drew says drive it, its learning.

 

[Jeff B]

Green 90 has done that. With the HD fan clutch it makes a ton of noise.
Drew says drive it, its learning.

I think I'll do that for now--- drive it and let it learn...


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[BarryO]

Drew says drive it, its learning.

^^^This. My 97 was doing some weird idle behavior when I got it back, that's disappeared more and more as I drive it. Just like the computer is learning what's "normal".

 

[Jeff B]

Misfire has not returned and gradually the high start up idle is getting shorter.
Lasting just less than a minute now


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