Physics of the Accelerator Pedal Hesitation/Pause Dilemma

ocduff · #1 (**permalink**) 09-27-2004, 07:41 PM

Well, I'm still sorting out some quirks on my 91 Spider - one that has me puzzled is that I've been having problems with the accelerator pedal. At first it was sticky and notchy and when I would press it down it would take an inch or more of travel to get a response from the engine.

Upon investigation I found that there is no "throttle cable" to lube, but more of a "linkage". I went ahead and heavily cleaned it and lubricated it. That smoothed out the stickiness issue. But I noticed that in the linkage that one of the two connector "ball ends" that attaches a rod to the throttle body, that the ball ends are riveted in. One is loose and causes some initial slack (ie the one inch of free play in the pedal feel). So this explains my initial free play of travel before the the throttle engages.

But the mystery is that I'm still getting a very annoying and tricky to control hesitation or maybe a half second or a second that makes it very hard to change gears smoothly. It's also tricky not to stall or overrev the engine from a light because I'm constantly trying to "play" this hesitation and at this point it has me beat.

I've tightened up all the hoses, the engine idles and runs very smoothly, it's just this pesky hesitation. I don't think it's the loose "ball end" though I haven't totally ruled it out (I think it isn't because even if I'm in, say, 4th gear at 3500 RPMS and I have the accelerator depressed 3 inches down, keeping a steady speed (meaning the slack in the loose linkage is already tightened up), when I go to accelerate more I still get that hesitation. It's not a free play feeling - more of a delayed response of a half second.

The spark plug wires look new (no cracks visible, even with engine running in the dark), the plugs are new Platinums, the air filter is pretty nasty but everything else looks good. I run 93 octane and I have Techron cleaner in there now. New air filter on the way.

I'm going to look at replacing that ball end (ugh - it's hard to get to, below the intake manifold). Not sure what part it is or how to go about replacing it - but as I said, I'm not sure this is my hesitation problem - or is it? Anyone have any advice?

JoeCab · #2 (**permalink**) 09-27-2004, 10:15 PM

I've been dealing with the same issue on my high mileage '91 Spider for some time now. I traced it to two culprits:

1) Throttle rod linkage ends - the plastic connectors that snap over the ball. I replaced these with very nice metal linkage ends we stock at Centerline and this removed *A LOT* of play from the linkage. Huge difference.

2) Mis-adjusted TPS. Adjusted per manual, also cleared up the last bit of hesitation. This has a lot to do with the throttle response coming off idle.

Joe

ocduff · #3 (**permalink**) 09-27-2004, 11:24 PM

Joe - thanks for the response and tips. What is TPS (I'm a newbie here)? I'll have to adjust that (I'm waiting for my cardisc). And I'll go ahead and order some of those metal linkage ends, but first I have to address the fact that the actual ball that they snap onto is loose in the linkage. It's not so much a problem with the snap ons (I wish) - it's the actual part that it snaps onto. Not sure where to get them or how to install them.

Johan · #4 (**permalink**) 09-28-2004, 04:40 AM

Throttle Position Sensor perhaps???

MrT · #5 (**permalink**) 09-28-2004, 04:51 PM

You might be able to tell which of the two items (if any) is your primary culprit by opening the throttle by hand under the hood (car at rest, out of gear, and so forth....). If your problem is the throttle position sensor, then you'll get the hesitation problem. If you don't get the same problem, then maybe the ball joints are your major villain. Of course, there are conspiracies afoot of late.

Michael

JoeCab · #6 (**permalink**) 09-28-2004, 05:40 PM

Yes, TPS is the throttle position sensor. There are specific procedures to test it in the manual, but the easiest test is you should hear a click coming from it when the throttle is moved just a few degrees off the idle stop. If you don't get that click, it is adjusted incorrectly and can cause the off idle hesitation.

Joe

ocduff · #7 (**permalink**) 09-29-2004, 02:06 PM

I adjusted the Throttle Position Sensor this morning - needed about a half turn clockwise to get to the point where it would just begin to raise the idle RPM's. I turned it to the point where it was just about to increase RPM's and then backed it of 1/8th turn. This helped my hesitation immensely - but will need further improvement by replacing parts in the throttle linkage. Also, I think a new air filter will help (previous owner didn't mind them black and full of junk). I ordered a K&N and then read up on them a little about their filtration levels and decided to go with a Purolator paper filter. Better safe than sorry.

**Gubi** · #8 (**permalink**) 09-29-2004, 02:14 PM

Yeah, steer clear of the K&Ns. At best they're maybe good for a couple HP at full throttle, but they let way too much crap through.

**Sniady** · #9 (**permalink**) 09-29-2004, 07:37 PM

Arrrr I just wrote up a big ole thread, and it didn't go through... ARRRR. I write it up again, but quick.

Concerning your problem.

First check your boot for cracks (the one from the air box/air mass meter, to your throttle body), take it off and inspect it. A small crack will cause a stumble.

Second... I explain how the TPS works. Its a potentionmeter which is a circuit with a arm with brushes on it, and as your hit the gas the arm moves up and down, increase/decreasing volatage and resistance. Well the tps, and air mass meter work together. They both send signals to the engine control module, and that calculates the air/fuel mixture, by measuring the throttle postion from the tps, and the air flowing from the air mass meter (air mass has a potentionmeter aswell). These potentionmeter brushes on the arm wear out after years, which create flat spots, and stumbles/hesitations.

Alrighty... so check that boot, and if it turns out to be intact. I try and feed you some info on how to measure the volatge and resistance at the tps, and air mass meter. I just dont feel like typing it twice.

p.s. sorry if there any mistakes, i rushed through this

MrT · #10 (**permalink**) 09-29-2004, 08:37 PM

As the man says, cracks are bad. The motor twists on the mounts when the throttle is punched, and cracks in the boot can open temporarily and lean out the mixture, killing power. Just thought I'd mention the transient motion as something that opens the cracks more. I like to have a plausible mechanism when I am troubleshooting things. Just looking around everywhere in general in hopes of finding something out of whack with a mechanical system I don't know intimately is not very attractive to me.

Michael

**Craig** · #11 (**permalink**) 09-30-2004, 01:45 AM

G'Day All,

Sorry Sniady but the bosch motronic 4.1 uses a switch type TPS. That is a unit containing two switches, one for idle and one for Wide Open Throttle.

JoeCab had the right info on adjusting it but the following links are to the pages of the 75 TS manual which describe it.

http://www.users.on.net/~craigf/pics/75TS_S-04-28.gif
http://www.users.on.net/~craigf/pics/75TS_S-04-29.gif

The images are really big but should print ok.

ocduff as others have said, an air leak could still be your problem.

ocduff · #12 (**permalink**) 09-30-2004, 12:17 PM

Craig - thanks for the pages, but do those instructions apply to the S4 Spider?

**Craig** · #13 (**permalink**) 09-30-2004, 02:00 PM

ocduff,

If the series 4 uses bosch motronic 4.1 as I believe then it will have a switch type TPS and the basics of the instructions will be correct even though their will be differences it the throttle body.

As JoCab said it should be adjusted so that you hear a click as the butterfly just opens and another approaching full throttle.

Measuring resistance as in the manual pages will tell you if you have a faulty switch.

More information on the motronic system is in section C of the Alfa 75, Electrical & Electronic Diagnosis manual on my site http://www.users.on.net/~craigf/ but you have to read between the lines as it refers to the use of an Alfa test box.

For others, Section D covers the L-Jet system.

Paradiso · #14 (**permalink**) 09-30-2004, 02:17 PM

Yes, S4 and TSpark both use Motronic 4.1. And yes, the TPS is an On/Off switch.

ocduff · #15 (**permalink**) 09-30-2004, 03:43 PM

Well, thanks everyone, I'm going to mess around with it this weekend and see if I can't get it mostly straightened out. The car is running fine, just trying to isolate that last bit of hesitation (seems to go away somewhat if I adjust the idle screw a little high ((I'm assuming that's what the small brass screw is on the throttle body - unless it's an idle mixture screw)). Every time I give the car a little TLC it seems to help...

Not sure that I'll be able to figure out the throttle linkage this weekend without the parts - but there's plenty more to do: change/flush fluids - also, find out about my NJ DMV inspection station REJECTION because of an "exhaust leak" on my car. It's an Ansa exhaust with no center muffler and there may be a few small holes in it, but I'm not convinced there are. It's just louder than a normal exhaust. Any good ways to patch it if there are any leaks? This may also be part of my problem but I really don't want to drop a $1000 on a cat back exhaust - everything else passed, it's just the leak that got me.

Thanks again to everyone for their input and advice. I have the needed info to look into the TPS.

-Owen