transmission issue?

[DavidT]

Hello everyone.
I noticed something today I have never noticed before. I was driving down the highway cruising at ~60 mph. I let off the gas as I approached a car. I FELT something kinda like a downshift or a studder or something... possibly the feel of applying the brakes.
So I tried it again... I got up to 60 looked at the rpms- about 2100 or so. So I let off the gas again. Same thing happened... then the rpms dropped to 1500 as the speed dropped to about 55. I tried it again... same thing happened- rpms dropped from 2000 to 1500 a few seconds after letting off the gas.
So I turned OFF the OD. This time the rpms were obviously higher, but it did the same thing... rpms would drop around 500 rpms shortly after letting off the gas pedal.. also FEELING a studder kinda like I am applying the brakes... then the rpms drop.
I had a full tranny rebuild back in October. I am also using Doug's Xcal tune.
Anyone have any ideas what she's doing?

 

[Mr Anonymous]

Your sure this is new? Have you gotten a new tune recently?

It sounds like aggressive engine braking, which can be configured in a tune.

 

[DavidT]

its pretty noticeable... I don't see how I've missed it for the last few years. It did not do it today much, maybe once or twice. Could it any way be a transmission issue? Torque convertor maybe? The rpms dropping so low puzzles me.... its as if the tranny has found an extra gear. I mean how in the world can I get only 1500 rpms at ~53 mph?

 

[mustangmann]

I have noticed this before also a long time ago on my 98. Now granted my car has 223,000+ on it with the original tranny but it does this exact thing too. When driving on the freeway for example and let off the gas to hit the off ramp, right about 60-65 mph is when it does it. It feels like an "engine brake" and you would think the rpm's would go up but they go down to like 1200-1500 like he was saying?? They then pop back up to normal rpm range when coming to a stop! Wierd, but I have just delt with it! My tranny also shudders alot right now so I am just trying to milk it out as long as possible! Not really worth me getting a tuner to help anything anymore! Oh well.

 

[SHOZ123]

There is a PCM strategy called coasting fuel shutoff. This shuts off the fuel injectors while coasting, usually after a few seconds of closed throttle and at highway speeds.

I change this when writing a tune and am much more aggressive with it. The TC can be made to stay locked up too at this time.

 

[stangeater]

Could be a shift silenoid going bad. Had that once.

 

[Mr Anonymous]

I checked mine on the way home tonight just to see what I'm turning for RPM's at 55 when coasting. I get about 1700, and that is with the converter locked.

 

[DavidT]

wow...
ok, so I am at 47 mph at 1700 rpms... UNLESS it does what I have described. Is this part of Dougs tune or is the tranny going nuts?????
Chris, are you tuned OR stock?
How do you know the TC is locked? Could it be that I am feeling the TC lockup?

 

[DavidT]

Well, it apparently is a tuning issue. I returned it back to the stock programming. Now the rpms drop immediately upon letting off the gas, there is no longer a delay.
From what I read on the net, the TC should disengage/unlock as soon as you let off the gas or hit the brakes. So Chris... isn't the TC UNLOCKED when you are @ 1700 rpms at 55 mph?

 

[Mr Anonymous]

Well, it apparently is a tuning issue. I returned it back to the stock programming. Now the rpms drop immediately upon letting off the gas, there is no longer a delay.
From what I read on the net, the TC should disengage/unlock as soon as you let off the gas or hit the brakes. So Chris... isn't the TC UNLOCKED when you are @ 1700 rpms at 55 mph?

It shouldn't be unlocked, since I tell my converter in the tuning to basically stay locked in 3rd and 4th unless I really step on it. BUT, I'm also having a lockup problem with this converter, so I can't say 100% that when I checked the other night that it was in fact still locked. Now that the warmer temps are here, I'm losing lockup just about every day after about 20-30 minutes of driving.

I probably should have said "and that should be with the converter locked."

 

[Mr Anonymous]

Out of curiosity, I re-enabled coasting fuel shutoff (engine braking) today, and on the way home observed that at 55 with closed throttle and engine braking slowing me down, I was showing 1400 rpms.

 

[stephen newberg]

So 1400 vs 1700 with engine braking disabled. Useful data.

pax, smn

 

[SHOZ123]

You can get really aggressive with the engine braking. If you set things up right you can keep the FIs shut off as you manually down shift all the way into first gear. The key is to keep the rpms up above the cut off point. This is ~ 1400 rpm in my tunes.

Combine this with keeping the TC locked when coasting in 4th and third it will really slow you down.

 

[DavidT]

ok, so what is happening when you let off the gas, in other words, what causes the rpms to drop approx. 300 rpms (or 600 rpms with Paul's tune)? Is it the TC disengaging, or engine brakeing/ coasting fuel shut off, or both?

 

[eli terry]

Man does that sound like my car was doing a couple of months ago. Turned out to be a faulty fuel pump? And a non pressurized gas tank which was rusted and was replaced!

 

[SHOZ123]

If the TC stays locked there will be very little rpm drop. If the TC is unlocked and slipping there will be more slip and rpm rise.

 

[DavidT]

Man does that sound like my car was doing a couple of months ago. Turned out to be a faulty fuel pump? And a non pressurized gas tank which was rusted and was replaced!

were you getting the p1450 check engine light? Because I DO get that every so often...

 

[kcobra]

What part# or what is the transmission in the 96-99 SHO's? Is is like a 4r70w, etc...

 

[SHO#7]

What part# or what is the transmission in the 96-99 SHO's? Is is like a 4r70w, etc...

AX4N.

Mike