Unintended Acceleration Yesterday - GT40

[John]

I have to admit that I've always been skeptical of the unintended acceleration claims that have popped up since, when was that, the '80s?

Anyhow, it happened to me yesterday. I was doing some spirited driving on a twisty road. I had recently had the car tuned and had been in the high rpm range, staying in an area where I hadn't spent much time before. I caught up to a 280Z at a relatively short passing section before a right hander that would require light braking.

I was next to the other car at 6,500 when the pedal went to the floor. I depressed the clutch - the engine went to its 7100 rpm governor - put the car in neutral, and before I even got my hand off the shifter to reach for the key, the pedal returned to my foot and the engine went toward idle.

I slowed down but continued driving to where I could safely pull off the road. The car behaved normally even up toward the 4000 rpm range except that it dropped toward idle between shifts slower than normal. The pedal felt normal, it was free and there was no stickiness in how it felt. It traveled smoothly.

When I stopped to look things over, nothing was amiss. The car has a TWM induction system controlled by a Big Stuff 3 ECU. Apparently, there's a throttle return spring in each of the induction ports although I haven't seen them personally. The pedal has a return spring too. The return spring on the pedal was still attached and intact. I didn't look at the induction system since everything felt and sounded normal.

I got back into the car and started off slowly for home, operating conservatively as if I was in gas saving mode. When the engine reached 2000 rpm though, it made a sound like a belt was slipping and the engine immediately lost power, as if it had reached the governor. I lifted and when the engine dropped below 2000, the noise went away and the engine ran normally.

In playing with the anomaly and investigating it, I could get past the noise and power loss by accelerating through the early 2000 rpm range at perhaps 20% TPS. That's just an estimate as I sit here thinking about it. However, when the engine got back into that range, 2000 - 2200, the symptoms showed up again. I only accelerated past it twice.

As I drove home, the anomaly became less frequent and it eventually stopped all together. As it became less frequent, so did the slow-to-idle-between-shifts issue. On the way home, I stopped for coffee and the car sat for half an hour or so. The rest of the drive was perfectly normal.

Any ideas?

 

[Howard Jones]

.....

 

[John]

Yeah, Howard, I know. The trouble with that, and me, is a combination of aesthetics and fear. The TWM looks like Webers. Replacing the TWM with Webers makes me think I'd be spending all my time tuning - or tying to.

 

[Ron Earp]

Before you guys get out on a limb any chance the carpet caught and kept the accelerator down? Or your shoe got wedged and did the same? Or the throttle return spring(s) on the induction are not up to snuff? Or the throttle body linkage bound up?

As with any kit cars there are 1000s of ways to install this stuff.

 

[Bill Musarra]

John,
Don't know if this will help, but look at your motor mounts. The drivers' side may be broken or cracked. The engine will try to flop over due to torque. The resulting rise in the engine will pull the throttle cable to wide open. This is seen more commonly in mechanical linkage than cable. If the cable is secured to a fixed frame item the resulting stretch will result in it acting like a section of linkage and the motor movement will pull the throttle wide open. The cable housing may be stretched as a result causing some slack in its mounting positions along its path if you have it mounted to the frame and to the engine. The sound you heard may be the cable rubbing a belt or some moving item like the alternator or the a/c compressor. Had it happen to me in an old 60 Pontiac station wagon going around a left hand corner under normal acceleration. Did it right in front of my father. He thought I was hot rodding it!!

Bill

 

[Ian Anderson]

You mention you were at high rpm for a while
Heat buildup causing the throttle cable to "tighten up" in the sheath and causing it to drag.

Add another spring on the pedal return - making sure it also pulls the cable with it. and take the cable out and relubricate it

Ian

 

[Mike Huddart]

The TWM system has one coiled return spring per throttle body, ie one per pair of runners. That's still four hefty return springs, though, plenty strong enough and the way they are set, fool proof.

There is mention of a screeching noise, could that be a huge vacuum leak, do the revs drop quickly, as normal, when you shut off, do you have an IAC motor. If these malfunction or the ECM tells it to malfunction they could pass a lot of extra air, maybe not enough to send the revs soaring but that screeching is curious.
Mike

 

[John]

The screeching noise is curious and so is the mechanical malfunction. I can't see how it could be tied to the ECU but I haven't come up with anything logical yet so why not blame that?

To answer questions . . .

Ron, the carpet was not a factor nor was my foot. Curiously, the pedal went to the floor when it happened as if that was the default position if a spring should break. The throttle return springs on the system are actually quite stiff and remain so after the event. I didn't see any issue with the linkage when I inspected after it happened.

Bill, I'll take a look at the motor mounts. Good suggestion.

Ian, since the throttle return springs are robust, edging toward too robust, I'll leave them as is but I'll follow your suggestion to lubricate the cable

Mike, I don't know where the vacuum leak could come from. Initially, the revs were slow to drop during shifts but went back to normal after a time. Doesn't the IAC keep the engine at high idle when it's cold? If so, I don't have one.

Thanks for the help, y'all. I'll do some inspecting tomorrow and some posting afterward.

 

[Ron Earp]

You don't have a stepper motor controlled throttle assembly so something mechanical caused the throttle to open or remain open.

My guess it is cable related. We had a malfunction like that at this years GRM Challenge with the Lola. One driver was going out of the paddock and blipped the throttle and it remained open (not WOT) as the cable broke/failed due to heat on the cable and the nylon sheathing melting and fusing with the braided inside. We couldn't get it fixed in time to run the rest of the day either.

 

[Mike Huddart]

John,
yes, an IAC speeds things up when the motor is cold, compensates for Air Con etc, sounds like you don't have one. You should get one of our TWM systems that come with an IAC, you would love it. I have never been to New Mexico, we do house calls too. ( joke! ) ( sort of )
Don't discount a vac leak with loss of power, have a good look 'round the motor and see if you can think of something. We had a race customer once with a blown base plate gasket and I could here the screeching up in the stands.
Mike

 

[Mike Pass]

Do you have any heat shielding round the cable at the engine end? I have mine wrapped with a tube of heat reflective/insulating fabric. If the cable getts cooked by the exhaust which is very likely on a 40 then the inner sheath can melt and cause the cable to stick. When it cools down the plastic would solidify again and then a few moves of the cable would gradually free it off - until the next time you fry it! Thsi seems ti fit with your symptoms except for the screaming noise - was that you?
Cheers
Mike

 

[Ron Earp]

Do you have any heat shielding round the cable at the engine end? I have mine wrapped with a tube of heat reflective/insulating fabric. If the cable getts cooked by the exhaust which is very likely on a 40 then the inner sheath can melt and cause the cable to stick. When it cools down the plastic would solidify again and then a few moves of the cable would gradually free it off - until the next time you fry it! Thsi seems ti fit with your symptoms except for the screaming noise - was that you?
Cheers
Mike

This is a much better description of what happened to us at the track and what I think is happening to you.

R

 

[Al Wohlstrom]

I have to admit that I've always been skeptical of the unintended acceleration claims that have popped up since, when was that, the '80s?

Anyhow, it happened to me yesterday. I was doing some spirited driving on a twisty road. I had recently had the car tuned and had been in the high rpm range, staying in an area where I hadn't spent much time before. I caught up to a 280Z at a relatively short passing section before a right hander that would require light braking.

I was next to the other car at 6,500 when the pedal went to the floor. I depressed the clutch - the engine went to its 7100 rpm governor - put the car in neutral, and before I even got my hand off the shifter to reach for the key, the pedal returned to my foot and the engine went toward idle.

I slowed down but continued driving to where I could safely pull off the road. The car behaved normally even up toward the 4000 rpm range except that it dropped toward idle between shifts slower than normal. The pedal felt normal, it was free and there was no stickiness in how it felt. It traveled smoothly.

When I stopped to look things over, nothing was amiss. The car has a TWM induction system controlled by a Big Stuff 3 ECU. Apparently, there's a throttle return spring in each of the induction ports although I haven't seen them personally. The pedal has a return spring too. The return spring on the pedal was still attached and intact. I didn't look at the induction system since everything felt and sounded normal.

I got back into the car and started off slowly for home, operating conservatively as if I was in gas saving mode. When the engine reached 2000 rpm though, it made a sound like a belt was slipping and the engine immediately lost power, as if it had reached the governor. I lifted and when the engine dropped below 2000, the noise went away and the engine ran normally.

In playing with the anomaly and investigating it, I could get past the noise and power loss by accelerating through the early 2000 rpm range at perhaps 20% TPS. That's just an estimate as I sit here thinking about it. However, when the engine got back into that range, 2000 - 2200, the symptoms showed up again. I only accelerated past it twice.

As I drove home, the anomaly became less frequent and it eventually stopped all together. As it became less frequent, so did the slow-to-idle-between-shifts issue. On the way home, I stopped for coffee and the car sat for half an hour or so. The rest of the drive was perfectly normal.

Any ideas?

I sold a lot of TWM setups as well as having one on a 496" 427 SO in a Kirham Cobra. I never had that problem. I always sold FAST ECUs with the setups. I suggest you look at the ECU.

 

[John]

Al, I was thinking the ECU too but how would it get control of the mechanical linkage to the throttle? The pedal went to the floor. On the other hand, the other curious thing is the loss of power that accompanied the screeching sound. I can't make any sense of the connection there. The loss of power felt like a governor which IS controlled by the ECU.

 

[Ian Clark]

Another application of Murphys Law. We've changed our throttle cable housings from PTFE lined to spiral wound steel. This seems to be the most resistant to heat and is a better fit with the stainless steel cable on our side pull throttle kits.

Throttle springs on the carb and pedal are very important no doubt. What is surprising is the internal drag on the cable created by the tension. More springs can work against you, resulting in a very stiff, hard to modulate throttle. Getting it right is a trick.

Throttle cable can also loop or snag causing hang ups. Or as Ron pointed out carpet and trim can get in the way.

Also the cable sheath can back out of the terminating ends at the brackets which will hold the throttle open by a length equal to the amount of sheath recessed into the cable housing end.

All of the above can be pretty hair raising, a rev limiter is a must, plus an easy to turn off ignition or master battery switch.

 

[Tom]

John,
If I recall, your ECU should have some data logging capability. If so, is it too late to download the data from your last drive and strip chart it? Might give you enough clues to narrow down what happened (i.e. throttle position, RPM, A/F ratio, vacuum...). Also, is there a capability to perform a BIT check on all your sensors and see if they're working right?

 

[Pat Buckley]

You don't have a stepper motor controlled throttle assembly so something mechanical caused the throttle to open or remain open.

My guess it is cable related. We had a malfunction like that at this years GRM Challenge with the Lola. One driver was going out of the paddock and blipped the throttle and it remained open (not WOT) as the cable broke/failed due to heat on the cable and the nylon sheathing melting and fusing with the braided inside. We couldn't get it fixed in time to run the rest of the day either.

A loose battery ground can cause the accelerator cable to get hot fast if it becomes a ground cable...

It happened to me at Brainerd in a Formula Ford. Sucked.

 

[Kirby Schrader]

John,

I see many others have made lots of good suggestions, but let me make one other suggestion since the same thing happened to me. Yes, I have EFI, but in this case, that would NOT have made any difference. Same thing would have happened with a carb.

Have a look at where the throttle cable goes through the pedal box. I looked and looked for a problem and only found that in rare cases, the adjustment screw on the end of the cable could actually catch on the edge of the hole in the pedal box.

I ground off the sharp edge of the hole (bevelled) and also ground off a little bit on the cable end. So far, no problem and I could not reproduce it like I was able to initially.

I know, I know... Off the wall, but hey... I was in second gear at the time and I did NOT want to be going as fast as I was...

If you need pictures, let me know and I can get more detailed.

FWIW,
Kirby

 

[John]

Thanks for all the advice.

Well, I didn't have time to investigate today. Chores got in the way.

CobraJet, my ECU can only log date when it's hooked up to a computer so no luck there. I don't know about a bit check. I'll have to call the manufacturer.

Ian and Kirby, I'll do a close inspection of the throttle cable from end to end.

Thanks again for the help.

 

[Cliff Beer]

I had a bad case of "unintended accelearation" a few weeks back...just when I was pulling onto the freeway behind an M3. The freeway was deserted, and for some strange reason the pedal floored itself and I shot around the M3 at the same time he seemed to be having a similar case of unintended acceleration going on.

When I looked down my foot was curiously on the pedal. As soon as I removed my foot the engine returned to idle so I assume it fixed itself.....