Loping between 1500 and 2000 rpm

[Takophiliac]

If this is obvious, my apologies in advance, tried searching but terminology to describe this didn't seem to return anything.

Please consider the following video:

You must be logged in to view this image or video.

I noticed this behavior not long after buying the car almost 6 years ago. The car is nearly stock, and has only a Fidanza lightweight flywheel, evo mr bov, injen intake pipe and k&n filter element as mods. Very nearly bone stock.

Any time the car is at low throttle and between about 1500 and 2000 rpm, I get this behavior. I don't normally drive in that range, so usually I only see it when I'm revving slightly (listening for lifter rattle, for instance) but occasionally I'll be in traffic in 5th and let it slow down enough to line up the rpm. In the case of being in gear, the loping can be much much slower (up to 10 seconds per cycle rather than the 1-2 shown here in neutral)

Before it's asked, the cel is for catalyst efficiency below threshold. (Used to burn oil leaked from exhaust valve seals, cat was never replaced)

Any thoughts on what might cause this?
Is this normal?

Edit: Fixed thread title.

 

[steve]

It sounds like idle surge except I can see that it's not doing it on it's own which makes me question the IPS operation. Normally once you open the throttle the IPS turns off and the ECU know not to manage the engine speed anymore but it this case it is acting like the ECU thinks it should and once the rpm's get "too" high it cuts fuel. It also look like with a bigger throttle opening the IPS does kick in and the ECU no longer does it.

So what year car is this? On 1G's the IPS is a switch on the TB and on 2G's it's part of the TPS.

 

[Takophiliac]

My apologies, it is a 1995 (february) talon.

 

[Colt4G63 also]

What Steve said... Since you have a 2g, I would say bad TPS or improperly adjusted TPS. The ECU thinks the throttle is closed, so it's trying to control RPM accordingly.

 

[Takophiliac]

I'll have a look at it and report back.
I was under a wrong impression as to how the ECU determined throttle closed. (I believe I must have read the 420a operation notes at some point, and remembered that instead of the 4g63T operation notes.)

 

[Takophiliac]

Sorry to revive this after so long. Just wanted to update. In my case, the throttle closed switch in the Throttle position sensor was bad. The throttle angle sensor part of it was ok, hence the mis-intepreted output.
It appeared to work properly after adjusting for the throttle closed sensor, even though it reported 7% open with the throttle closed. It did did properly detect the closed position as well even at 7%.
I have since replaced the sensor with a working one, and haven't noticed any change.

 

[steve]

The 2G TPS couples the position and idle switch sensors inside the TPS. A 2G TPS must be adjusted per the FSM based on where the idle switch switches. The position voltage at that point has a wide spec range and the factory ECU code will then scale the values based on what it sees.

Is the injen intake pipe pre-turbo? Most of the time I see that it's on a NA car.

 

[Takophiliac]

Maybe i didn't state it clearly.
I appreciate the re-emphasis of how the 2g TPS works.

I adjusted (per the FSM) the one which was on the car in the video, and was able to get a properly working system despite the TPS indicating 7% open. Presumably, the idle contact had worn/corroded/shortend/moved but it didn't adversely affect the car after adjusting it by what must have been about 6 degrees to make the idle switch work properly.

Furthermore, after replacing it with a properly working switch (throttle position at 0% when idle switch closed) I didn't notice any difference from the broken adjusted switch (trottle position at 7% when idle switch closed). So I assume, based on that comparison, the throttle position sensor part of the assembly isn't terribly important or that 7% was an unnoticeable difference to the driver. So I wouldn't necessarily just throw out a sensor which needed similar adjustment to get the idle switch in the right place, unless someone can identify an issue which that causes.

The problem is resolved. It was a bad/misadjusted TPS sensor. Adjusting it resolved the problem. I just wanted to post back my observations and results so that anyone else searching for a similar problem may benefit.

This car has the injen intake system (like this: http://www.streettunedmotorsports.com/parts/injen_2g_dsm_intake.htm), it was on the car when i bought it. It is pre-turbo, of course, and i'm positive that the pipe itself is solid and not leaking or poorly attached (coupler is in good shape as well on both ends, all hoses are attached all hoses hold vacuum except the evap canister line).