Subject: 1987 IROC-Z 350TPI/700r4
Problem: Car will not freewheel

History: Car was bought in 2000 from florida. Pretty much everything on the car was original. The car is slowly being restored, but one problem remains since car was purchased. This car will NOT freewheel. By which I mean, if you take your foot off the gas while the car is going downhill, the car does not gain speed. The tach actually shows the engine RPMS DRPOPPING by 200-300 and then jumping back up. Depending on the length of the hill, it may do this several times. You can actually feel the car start to slow down the second you take your foot off the gas. You do have to be going more than 40mph and be in 3rd or 4th gear before the problem shows up. Now, before you jump in and say torque converter, I am going to list all of the things that have been replaced on the car.

2000 MAF replaced. FANS replaced. Fuel filter.
2003 - tranny totally replaced. Including torque converter. New brake switch.
2005 Fuel pump. Brakes completely redone.
2006 Alternator replaced. Entire gauge cluster replaced.
2007 ENTIRE engine replaced. Not rebuilt, replaced. This included things like EGR, TPS and some new wiring.
2008 Reman computer

NONE of these things has made any difference in the symptoms whatsoever. I have a shop with a cumulative total of 60 years of CHEVY experience working on this thing, and they are totally stumped. The owner and chief mechanic SPECIALIZES in camaros/firebirds/corvettes and has owned probably close to 20 of them, including 5 3rd gens. He's never seen anything like it. He can get it to duplicate but he doesn't know why. He set up a BUNCH of voltmeters all over the car, and a diagnostic computer, and the ONLY thing he can see is that when it happens, the engine is really going lean. (fuel) But we have no idea WHY this should happen. They even played with the adjustment on the TPS and it didn't change a thing.

Has anyone ever seen or heard of this before? We are truly out of ideas. I love driving this car but this is a ROYAL pain in the mountains, and I live in Colorado! It makes the cruise control beserk, and makes it very difficult to maintain a consistent speed on ground that is not majorly flat. The only change we have EVER seen in the symptoms is that it got more pronnounced with the new engine, because we had better compression etc. The old engine at 140k was pretty worn out. I'm really looking forward to some road trips this summer, but not if we can't get this fixed!

Please help!

 

Yea, it sounds like it's stuck in 3rd gear with the torque converter locked. I mean, I would assume this is purely tranny related.
If the engine runs well, I would not think it's related to engine, or electrical at all.

When you take your foot off the gas going down a hill, your vacuum level will jump up and you will go lean. That's just normal.

- What happens if you're going down a hill, and pop it up into neutral? i'm seeing if your gears are too tight?

- does it cruise on flat ground (with your foot on the gas) pretty predictably? ie normal power, etc?

 

No, it's not stuck in 3rd gear. All 4 gears are there

I've had 3 different tranny shops look at it. They all agree its not the tranny. Or the torque converter

If you put it in neutral going down hill, it rolls just fine. ALSO, if I am going downhill and I just push on the brake pedal enough to activate the switch, but not really the brakes, it'll freewheel.

Yes it cruises on flat ground as long as you have your foot on the gas. If you take your foot off the gas you can feel the car slow immediately. As though engine braking were being applied.

As you can see, this is rather strange.....

 

I agree it's defenatly the T/C locking up, when you apply the brake it automaticly unlocks the T/C, there is an eletrical connector on the front of the trans above the pan facing down...I think in the pass. side, it is for the T/C, try disconnecting it and see if it still does it, if not there is a fault in that circut and it needs to be troubleshot, don't leave it disconnected too long it will kill your gas milage and possabaly give you some trouble codes

 

Nah, I didn't mean if other gears were accessible, I just meant it's staying in 3rd gear, as in not freewheeling. The freewheeling is when the tranny lets the engine go back down to idle while the car.. "freewheels" and keeps rolling. Your car is acting more like a standard tranny car, it's locking the engine to the wheel speed. That's the tranny, not an engine problem or anything else.

I think lewandom is onto something there.

 

Ok - this is a NEW tranny and torque converter. It's doing exactly the same thing as the OLD tranny and TC. No change whatsoever. They put a meter on the TC circuit, it is NOT the TC locking and unlocking. (i know it sounds like it, but its not) The vacuum pod has been tested, the brake switch has been tested and replaced, the TC circuit has been tested. Its actually the ENGINE making the RPMS jump around, not the tranny. We replaced the ECM because we couldn't think of anything else that could be messing with it. That didn't change anything either. It's not staying in 3rd gear, we have all 3 shifts, and the RPMS at cruise tell us as well that its going into 4th gear. Not to mention, that if I manually put the tranny in 3rd gear, we have the same problem, with the same RPM fluxuation. We actually pulled the new tranny out and put in a non-locking torque converter in the car to see if we had something funky with the way the TC was being handled. All THAT did was make the problem more pronnounced.

Nice info, but this is gonna be something either much more oddball, or TWO problems somehow combining to make this happen. I'm beginning to wonder if this car came out of the factory wired wrong somehow.

 

hmm, well be sure to post the solution. Problems like this are why I prefer to avoid fuel injection, and other complications like that
I can't quite wrap my head around how the computer could be causing this, which is probably why i'm still running a carb.

 

Well, we know its NOT the computer causing it because we replaced it. No change in symptoms whatsoever. But SOMETHING is causing the rpm to flop around when the engine is not under load. It'll do it at 40mph, it'll do it at 70mph. The computer takes input from sensors to manage the engine, but the ONLY sensor that I know of that could actually directly change the RPM's, is the TPS. Which has been replaced. The ignition module, MAF, EGR, oil pressure sensor and coolant temp sensor have all been replaced. I'm running out of things to replace!