Part throttle misfires

[gixxer750]

hmmm...

I've got part throttle misfires at cruise speed at a steady throttle position.

Plugs are new, hell the whole engine is new. Plugs are 3923s gaped at .028. Compression ratio is 8.5:1 if that matters. Wires are new NGK wires. There is not oil in the spark plug area. All valve clearances are in spec, and no codes...

I do not have misfires on acceleration, but I do have a lot of poping on decel.

 

[aye mate]

Rich?

 

[figgie]

ooh easy one.

check your timing

you are probably close to zero degree advance

i bet the turbo spools up nicely

 

[TurboSupra7]

i tried tracking down that same problem on my car. I was so convinced that it was the TPS, despite everyone telling me that the TPS doesn't have that great of an effect on noticeable driving conditions. But eventually I tweaked it right to that perfect position and my car runs smooth as hell.

Anyways, check and recheck your TPS.

 

[gixxer750]

the tps hits a max of 3.4 volts, and .54 volts at 0 throttle.... that sounds dead on to me. Base IGN timing is at 12 deg BTDC

 

[figgie]

3.4 volts?

That tps is fubared!... should read close to 5 v at WOT!

 

[gixxer750]

hmmm.... I thought it was supposed to be 3.5

 

[jetjock]

I'm confused. If you think 3.5 is normal why does it "sounds dead to me"? Understand, I'm not saying 3.5 is bad nor am I saying misfire can be TPS related. Others are saying that. I'm only confused why you feel the TPS was dead one post and normal the next?

 

[gixxer750]

the tps hits a max of 3.4 volts, and .54 volts at 0 throttle.... that sounds dead on to me. Base IGN timing is at 12 deg BTDC

Not being a smartass, just bringing up what I said.....

So... TPS voltage max is 5.0 or 3.5? I have 2 tps and both max at around 3.5....

 

[jetjock]

Sorry, seems my eyes are going. Touche

The book says 4-5 but looking at how the thing is wired, knowing what's inside the ecu, and considering the wide resistance spec for measuring the TPS, 3.5 looks correct. Fwiw mine is 3.5 and works fine.

 

[MDCmotorsports]

This being brought up, I wonder if you can lean or richen out the fuel maps by making the WOT value for the tps more or less...

 

[jetjock]

Since the engine mainly depends on the AFM for calculating basic injection duration and since the throttle plate itself controls airflow the TPS is used as a trim and more as an accelerator pump than for throttle position. A more effective way would be to mess with coolant temp. Better still would be to manipulate the cold start injector.

I doubt his misfire is TPS related but he might want to unplug it and see if things improve. The car should run pretty much the same as normal. Might see a slightly higher idle but that's it. Emissions will go up slightly too. Misfires? Assuming what he's describing is a legitmate misfire I doubt his TPS is at fault.

 

[TurboSupra7]

I had the most irritating part throttle misfire you can imagine, and I just knew it had to be the TPS. About a week and a half ago, I finally found that perfect spot for it and, what do you know, no more misfire AT ALL. As the car gets old the resistance in the TPS gets all out of whack. Keep messing with it, not necessarily following the TSRM as a guide. Just take it a step at a time.

The TPS has more of an effect on part throttle driving than most people believe.

 

[gixxer750]

well, I found a completly different cause than anyone suspected.

Cylinder2 had the wrong valve shim in it, holding the valve open enough to give me 80 psi on cylinder 2. A leakdown test revealed that it was an intake valve causing the problem. The leakdown test showed 15psi/100psi....

Just for extra info, here's how my compression test came out. This engine is 8.4:1 comp ratio.

Cylinder 1 -167
2- 80w/prob -168 after fixing Leakdown after fix 93psi/100 psi
3 - -167
4 - -167 Leakdown 92/100
5 - -167
6- -166

 

[jetjock]

Not surprising as the second most common case for misfire (after ignition problems) is valve problems ie; clearance or a burnt one. It's also a common cause for high HC. It should never be the TPS though, regardless of what others have experienced. Just doesn't make sense based on how the ecu works. A least not in the sense of true misfires.