pflyers

My car has a problem where on deceleration where it feels like the engine is trying to pull the car to a stop, VERY annoying in traffic.

I swapped in a new TPS thinking that was it but nope.

I noticed today that my vacuum is a nice 15 lbs. at idle but when I decelerate it drops down to 22lbs of vacuum and causes that annoying problem.

Anyone have any idea what this may be or how to debug it?

951kid

Sorry i dont have a good answer but if you shift into a higher gear the effect of engine breaking will be much less noticible because of the effective torque of the engine, otherwise you might want ot check out your idle air injector i think its called, it monitors incomming air when your foots off the gas.

Anthony

Diver944

I've always found that my 951 does not have much engine braking, which is quite annoying when I lift off well before a speed camera, but then have to dab the brakes to stop getting a ticket.

My S2 has much more engine braking and I have always attributed this to the much higher compression of the S2 engine

sm

[QUOTE=pflyers]My car has a problem where on deceleration where it feels like the engine is trying to pull the car to a stop, VERY annoying in traffic.
[QUOTE]

Does this mostly happen in 1st gear above 1600 RPMs? IIRC, the car's DME shuts the fuel injectors off as you let off the throttle and reopens them once the RPMs dip below ~1600. This is for emissions purposes and is felt mostly in 1st gear, though the effects are less evident in the higher gears.

Let's see what the experts think...

pflyers

no it's mostly noticeable in 3rd gear

Bret 944

Weird, but I think I recall my non-turbo having similar symptoms when the clutch was on its last leg. Might be related, but I'm not sure.

Charlotte944

That's why its called "engine braking." I would worry more if the engine did not brake the car on deceleration.

I take it you are letting the clutch all thge way out on down shifts. Have you tried "slipping" the clutch?

pflyers

It's not like any other car I've ever driven, and did not do that before the turbo came out and went back in. That's the only concept I can think of to describe it. It's not a smooth deceleration as it surges a little tiny bit off and on as it's decelerating.

I don't know if the car is supposed to show 22lbs of vacuum on deceleration or if that's normal but it is doing that when at idle it's sitting very smoothly at 15-16 lbs.

hrk

One reason for abrubt cutting is ECU as mentioned earlier. If engine is running above 1600 or 1800 rpm and TPS sends signal telling ECu it is closed i.e. idle. fuel is cut off and engine speed drops abruptly. Use search in Rellist and you'll find cure for this drivability problem involving rotating TPS in a way that it never sends idle signal to ECU.
hrk

Charlotte944

First off, vacuum is measured in inches of Mercury, or Hg, so what you are seeing is 15-16" Hg at idle and 22" Hg under deceleration and both readings are "normal". I put normal in quotes because I like to see vacuum in the 18"Hg range.

When you stop and think about it for a minute, vacuum is supposed to go up under deceleration. The pistons are trying to pull air past the throttle plate, which should be closed, and the deceleration RPM will almost always be higher than the idle RPM.

As for the surging, now that you mentioned it, my 951 does surge a little under engine braking, but only when the engine RPM is at or below a normal shift point. I've only had the car since March of this year, but that slight surge was one of the first things that I noticed was different, .because my '87 NA doesn't surge at all.

As I see it we have three choices:
1. Don't engine brake.
2. Down shift to the next lower gear before the engine RPM reaches the shift point.
3. Slip, or "ride" the clutch during engine braking/deceleration.

I use #2.