Physics of the Accelerator Pedal Hesitation/Pause Dilemma

[ocduff]

Well - The TPS is adjusted correctly (clicks open after a few degrees), and it was set right before I took a look at it. I checked each hose very well and it all seemed good (even the air intake hoses), though I did not test them in water for bubbles. The linkage still has play in it, and I'll be addressing that next (thought I don't think that's it) - but here's the next clue in this puzzle:

The idle adjustment screw (not an lean/rich screw as I understand it), when I turn it to the point where itjust begins to increase idle, then back it off a little - then when I get in the car and drive the hesitation is gone. Then when I get out and turn the screw counterclockwise, the problem comes back. (My idle seems to have one set speed - I can keep turning it counterclockwise and nothing decreases below 800 RPMS - then I turn it clockwise a few turns, nothing, then I get an increase in RPM's and it starts running a little rich).

But the reason I turned it back down was because the car started running rich (you could smell it) and there was a gas smell in the oil (changed it). Plus I read that you shouldn't use the idle screw to compensate for other issues. Anyway, it cured my problem, but not the way I wanted to.

So the problem persists - and to more clearly identify it now - it's a stickiness when I initially press on the pedal when the idle adjust is turned down to where it should be. It's like I press down on the pedal and it's sticky and then it gives and the engine responds at the proper RPM for how far the pedal is pressed down after it gives. It's like I push on the pedal initially and it needs a little more pressure than usual (there's slight resistance/stickiness). Makes it impossible to make smooth shifts. As I said, I'm going to look into the linkage but I think that may help the slight free play, not the stickiness tho. The hoses need a closer inspection too.

[Gubi]

If the pedal feels physically "sticky" it's gotta be either the linkage or the throttle body. I'd go over the linkage with a fine-toothed comb. Also take a look into the throttle body at the back of the throttle plate and the area where the butterfly valve closes against the body...might be all gunked up, in which case a good cleaning would be in order.

You mentioned you've been operating the throttle by hand, so I think you would've noticed any stickiness there. If that moves fine, gotta be the linkage, I would think.

Checking for cracks is a good idea, but generally speaking they aren't as huge of an issue on the Motronic cars. If you have a little air seeping in the Motronic will adjust for it via the oxygen sensor (assuming it's working properly) and if they get large enough that the engine can't bring things into tune you'll get a check engine light.

Incidentally, you mention that you were able to adjust the engine to the point where it was running rich. If it was really running rich you should have gotten a check engine light...did this happen? Do you know if your O2 sensor is working properly? If this goes bad it can cause many a weird running problem.

Have you tried pulling codes on the computer to see what's in there?

[ocduff]

Gubi -

Thanks for your response...I'm still tracking down this little gremlin.

By hand the throttle works fine - no problems, no stickiness (and when the car is off and I operate the accelerator pedal everything is smooth too). And yes, at one point when I had adjusted the idle adjustment high enough to increase idle speed, it did run rich and the engine light came on for about a minute after which I turned idle speed screw down. It was obvious it was running rich because of the gurgly engine response and "rich" smell, but it ran smooth!!

O2 Sensor is working properly as well as I know - I just passed inspection with pretty good levels (and I had it set kinda rich when I went in). Though I do get an occasional idle flutter when coming to a stop light after a long period of driving I'm not noticing many other symptons (the idle is NOT silky smooth) but it's pretty smooth and the engine seems to be running smoothly and not sucking gas. Do you know a good way to check an O2 sensor with a digi multimeter? O2's are $$$$ for the S4.

Haven't checked the "check engine" codes. The linkage appears fine (and when the car is off the pedal doesn't stick or anything/ everything moves freely). Maybe the throttle body is gummed up a little, I'm really sort of baffled at this point. Maybe it is the O2 sensor?

Last question: is it normal when increasing the idle screw to get increased richness?

Thanks again...

[Gubi]

Well, bear in mind that I'm not an expert here, but I have done a fair amount of Motronic troubleshooting. Just trying to brainstorm and come up with ideas.

Weird about the pedal. So you feel a physical pedal stickiness, but only when the car is running? It's not just a lag in the throttle response?

This thread (from when I was troubleshooting mine) has a pic of the O2 sensor connector along the firewall and info for testing: O2 Sensor Connector Location? The fact that you were getting a check engine light when you tuned it rich suggests that the sensor is at least working. Probably not your problem, then, (especially if it's passing smog okay) but easy enough to check anyway.

(As an aside, yes, the three connector sensor is quite expensive with the Alfa plugs. However, you can just buy a generic three-wire Bosch sensor and crimp it to the existing wires/plugs. Saves you about $100 and works exactly the same.)

With regards to the idle screw, dunno. On the Motronic cars, my understanding is that the idle speed is controlled by the computer via the RPM sensor. So you don't turn the idle screw to adjust the RPMs directly, instead you adjust it to get a specified flow through the throttle in the closed position with the engine idling (you need a calibrated flowmeter for this). Cardisc has a procedure, but I've never had to do it.

I suppose if you cranked it all the way closed you could strangle the engine enough to make it run rich. Are you sure your idle speed actuator valve is opening correctly?

Any time you have Motronic running problems, it's a good idea to pull the codes. Since you've been changing a lot of things there may be a lot of them in the system (for example, when you riched it out you'll probably have a bunch of 1223s) so you may not be able to make head or tails of them, but it's a place to start. At the very least you can clear the computer and then drive it around to see if anything else pops up.

[Craig]

G'Day Again,

Just wanted to say I agree with Gubi on all points.

Other weird and unlikely things to check would be that the butterfly is not loose on the shaft and that the shaft bushes are not badly worn.

Rough idle probably means the injectors need cleaning (hmm this may explain your hesitation).

[ocduff]

Gubi/Craig/Alfa BB'ers -

Thanks for your replies and information, it's given me a lot of guidance to help me trouble shoot this problem.

Basically, upon very close inspection of the throttle linkage and fiddling with it every which way (to no good solution), I suspect that the PO might have been tinkering with it too. Basically all the ball ends are loose in the way that they are riveted in to the arms that are connected to the return springs. It makes slack and looseness to the throttle "feel" and must be causing the hesitation and stickiness. I believe that this is my main culprit and it is worse than I initially thought. It looks like I'll have to replace most of it (I don't even know a good way to get to it).

An annoying job, but I think that the car is otherwise running fine after checking everything over, just that there is no way to take up the slack without breaking the cables. If you guys know of a good way to replace the ball ends and there return spring mechanisms, I'm all ears. The one on the throttle body is the only good one, the rest are bent and loose. Maybe the PO had adjusted it too tight and when he jammed on the throttle they bottomed out and bent everything up.

Thanks again - I think this should be my first step as it needs to be addressed anyway.

[jclark]

Look at the 'balls' for flat spots - this happened to me, and the cure is to replace balls and female recievers ( heim joints ? ) at the same time. This makes modulating engine speed over speed bumps in the parking lot *much* less stressful...

jeff

[ocduff]

Jeff -

Thanks - I think that's what I'll probably end up doing. I just need to find the parts and know how (since the balls are riveted in) to install them and how to reach down there to get everything apart and back together.

I don't think the balls have flat spots - they are just loose in the linkage - like the rivet has loosened. I think I'll just try and find the parts to replace the linage too because I'd hate to replace the balls and find out that the linkage is worn too.

Jeff - how did you go about all that?

Thanks

[alfaloco]

Sagging motor mounts can also play into the intake hose stress and cracking situation. I had to remove the hoses on my '91 to locate a crack which only opened when the engine was revved. So don't just inspect from the outside and pronounce them OK! Check both the molded flex hose connecting the airflow meter to the plastic tube across the cam cover, and also the small round rubber hose at the throttle body. Cracks are better seen on the ID as you flex the part with your hands. When you reinstall the hoses, check underneath to make certain they slide onto their mating parts 100% so you get a good airtight seal when the clamps are tightened.

The TPS (throttle position switch) is easy to adjust with a VOM (aka "multimeter"). Carefully unplug the connector from the switch at the throttle body. The throttle plate should be closed, engine off. Using the ohm scale, clip one lead onto the center terminal. The lower terminal is the idle contact and the top terminal is the full throttle contact of the switch. Clip the other test lead onto the lower terminal and just barely crack open the throttle plate. You may hear a click, and the ohmeter will register continuity. If this is not the case, loosen the two clamp screws on the switch body, and rotate it slowly until you get continuity between the center and lower terminals with the throttle plate closed. Then, gently back the switch off until the contacts just open. Lock it down at this point with the two clamp screws. At this adjustment point, you will find that the full throttle switch contact closes at about 3/4 throttle position, which is correct. There is no independent adjustment for full throttle however, so once you have the idle setting correct, you have it essentially right.

The brass screw referred to on the throttle body is an idle speed screw, not mixture. Also, the air filter must be reasonably clean in order to effect any meaningful adjustments to the intake side of the engine, so I would not make any judgements at all until I had a new air filter in place. Lastly, check the inside of the airflow meter for oil contamination. This was mainly a 164 issue, not Spiders, but presumably a blocked vapor separator could dirty up your airflow meter.

Dave Jarman

[doctorpws2004]

I am having the same hesitation problems butonly when I start out cold. I'll give the 86 Graduate some pedal and no acceleration. My mechanic recently had to work on my throttle linkage; could it be that the TPS was somehow incorrectly set or damaged in this process? (Mechanic said that the throttle linkage was bound up or binding on something or whatever).

[alfaloco]

No, Paul, I don't see the TPS working any differently on a hot engine versus cold, it's either adjusted correctly or it's not.

Anyone with L-jet experience want to comment here? Cold start solenoid maybe, or does the L-jet use the same temp sensor as the later Motronic system (which I am accustomed to)?

You may get better response on this one if you actually make it a new thread, Paul. Just a suggestion.

[Sniady]

A TPS sensor is a variable resistor. High resistance will increase heat. So you can have changes when cold or hot.

They both have brushes as contacts, and these brushes wear. They will put out weak signals/flat spots to the engine control module. Which calculates how much air, throttle position are applied to deliver the right about of fuel.

OCDUFF

Find the specs for your TPS, and Mass air meter, and check that with a multimeter.

[Craig]

Quote:

Originally Posted by Sniady A TPS sensor is a variable resistor.