Odd Cruise Control Behavior

[Legacy777]

Since rebuilding the motor a second time and the 6spd swap I've noticed the cruise control appears to hunt. It will accelerate, let off the throttle, speed drops, it'll accelerate again, and continue doing this. It acts like those people that can't just hold their foot steady on the accelerator. I had thought this might be since I don't have a good tune on the car, but I am able to easily modulate and regulate speed with my right foot just fine.

So I guess I'm wondering if anyone else has experienced this. Additionally, I am wondering if the control logic for the turbo specific cruise computer is a little less sensitive and doesn't make quite as large throttle inputs vs. the non-turbo cruise computer. I'm running the MT cruise computer out of 90 Legacy. Lastly, I'm not sure if the throttle actuator wears out or vacuum pump is not working properly so instead of fine adjustments, large adjustments are made.

Next time I'm out, I'll try getting a video. It's very obvious when you look at the vacuum/boost gauge and speedo.

[rallyak]

My wife's car also dose that. I came to the conclusion that the rings has low friction (my car also did this when total seal rings were installed). It runs great, doesn't burn oil but is totally stock with the original rings almost 400k. When not on cruise every thing is normal.

Also when I replaced my total seal rings for normal rings this all went away.

[AWD_addict]

Probably not control logic throttle changes. Putting a cruise computer from a '91 turbo on my '90 didn't make a large difference in ability to hold speed.

[wtdash]

JC,
I've read about this behavior on another Subie forum (or maybe here).....If I find it i'll post back....can't recall WHAT was the cause though....NOT discounting 'rallyak''s suggetion, either.

TD

[Legacy777]

My wife's car also dose that. I came to the conclusion that the rings has low friction (my car also did this when total seal rings were installed). It runs great, doesn't burn oil but is totally stock with the original rings almost 400k. When not on cruise every thing is normal.

Also when I replaced my total seal rings for normal rings this all went away.

Can you elaborate what you mean by "total seal rings"?

The other comment is that my vacuum reading is only about 10" when at cruise since of the slightly lower gearing in the 6spd.

[rallyak]

Total seal rings is a brand name, and they are a low friction and one ring is gapless (it is almost like the oil ring,three pieces makes up one of the compression rings) they claim higher vacuum and better mpg. Which both are true 21 lbs of vac at idle and best mpg was 37 on the hwy. The down side of these rings are, they are very sensitive to a perfect bore. Mine burn massive amount of oil 1qt in 700 miles. I pulled them after 3000 miles because of the oil consumption.

With these rings it was hard to keep the car under the speed limit, it really like to speed but on cruise it would speed up and slow down all the time, except when pulling hills.

[Legacy777]

Hmm....that's very interesting...I hadn't heard of them. The rings I'm using are the standard rings that came with the Wiseco pistons. To my knowledge they are standard type piston rings.

[Tommio RS]

The only real cruise issue's that i have seen on Subaru's is as followed:

newer outbacks and leggys get a reflash for surging on cruise.

Have seen some older foresters with aftermarket systems on them with some lines that have fallen off!
These cars have some really ODD behaviors!

Some more mechanical things can do similar things also.

Like a very VERY small intake leak, vacuum
Injector issue, sticky
Spark issues
Anything really that is a drive ability issue

Also a bit one thats over looked by many.

Clogged exhaust!

Again not to rag on the foresters but they old ones like to get the cat codes. The car most of the time will drive fine but suck hard on the highway! Like ZERO power. But its hard to notice when city driving. This causes the cruise to buck around and stuff.

I dont think you have a clogged exhaust tho. Im just saying.


So to sum up what im saying.

Dont over look the easy things.

I have done that many MANY items.

Tom

[Legacy777]

Thanks for the suggestions Tom. Other than the cruise, the car drives fine...or as well as it can with the tune not being perfect. I know that I really need to get that sorted before I really start to tackle other things.

[kleinkid]

Josh, while on cruise control, since my 6 speed swap, DCCD Pro, install, have a slight hunting issue. While watching the boost gage, it will go from around -11 to -8, then back. Of course the car will speed up some and then slow down some. I just assume it is the chip in the DCCD Pro controller and how it relates to the TPS. I'm used to it now, originally annoyed me.

[Legacy777]

Hmmm, I never thought about it being the DCCD Pro controller. The DCCD Pro controller shouldn't affect the TPS signal. I wonder if it's more of a 6spd issue due to the different gearing.

What you described is what mine does.

[kleinkid]

The chip for our cars is designed to read the TPS voltage in a different way than the one for a 20G, remember. Might apply.

[Legacy777]

Again, the DCCD Pro is just reading the voltage, it should not be adding any resistance or doing anything to change the signal. It's essentially acting like a pressure gauge, except for voltage.

It may just be one of those things. I'll have to look into it more when I get some free time.....I've got none now.

[mjc1310]

I'd like to bring this discussion back up to see if anyone has made progress on the issue. I have a '90 n/a that has the same symptoms except the CC will completely stop after about a minute of fluctuation. My ABS light is also on and was told these two systems failing can be due to one culprit.

So has anyone made any discoveries?

[legsportwag]

My car does this if the boost is turned up... If I keep it on the waistgate it helps a lot.. I noticed it much more with my old 92 tw (td04).. Turning boost down with that car helped too.. I'm on a MBC and always have been... I run a dohc 22t with a tdo6 20g and have no issues even over mountain passes. Usually will hold 1-3 psi the whole climb as long as I'm not cranked to 20psi via MBC. My waistgate is set at 12psi.. Also I hold 3200rpm at 75mph. I can spool 20psi by 3750k easy on 5th gear load...

Maybe we all need big turbos?! Lol

[Legacy777]

I don't have any updates, sorry.

[legsportwag]

I believe its mostly due to boost pressure and mods in general. Our cruise modules don't know what to do with more power....

Out of all my turbo legacy's I've noticed that my swapped l is the best for reliable cruise... It usually keeps boost under 5psi no matter what the terrain is doing (hills and mountain passes , I'm in the rockies)

[oldscoobyturbo]

I too had the best cruise control function on my 93 turbo touring wagon 4eat and the rebuilt 92 turbo touring wagon 5spd ra

[Legacy777]

I did a little bit of testing this past weekend and still have the same behavior even after the tune. I suspect it's the cruise computer. I want to try a turbo cruise computer to see if that helps or not. I have a harness from a 92 turbo legacy that may have a computer on it. If not, I'll post something in the parts shed to see if someone has one.