Tinkering with my saab

I think I have the hesitation problem in my saab narrowed down.. but I want a second opinion.

it is a 91 with the 2.1 litre, Five speed trans.

Normally it runs great, starts up fine, idles fine, and 85% of the time gives me no problems.. what it does do is sometimes act like I shut off the fuel. It will literally flat spot, the revs will drop, and the car will slow.. unless I move my foot. Seems the moment I even shift the pedal, the engine comes back to life. The engine never dies.. if I clutch in when this happens, it will fall to an idle.

I know it is not ignition, as there is no backfire when it kicks in again (no extra fuel in the engine or exhaust) so after doing a complete tune up of all filters, plugs, wires, cap and rotor.. I am thinking it is the TPS.

I would like to add that when the engine does this, the check engine light will come on and will flash the 4 flash code for knocksensor.

Do you think I am on the right track for the Throttle position sensor? It seems to point to it as even tickling the throttle will bring the car back to life in an instant

Are you certain that the check engine code isn't for a missing load signal to the EZK (ignition system)? Here's why:

If the throttle switch fails, then the Jetronic ECU's load signal to the EZK would fail, which would cause a check engine light. I don't know enough about the system to know whether a bad throttle switch can cause the car to fall on its face -- my experience with the LH2.4.2 stuff is limited and the only real difference between it and the earlier stuff is the throttle switch.

If the car feels like the fuel is shut off, then maybe it's a bad crank sensor? The Jetronic ECU will not operate the fuel pump without a tach signal. If you're losing the tach signal, you're losing fuel. I don't know why shifting the pedal would change this, though, so I'm going to guess that, yes, a bad throttle switch can cause your symptoms.

If you are going to the junkyard for a used switch, note that you cannot use a throttle switch from ANY 900 Turbo or ANY MY90 or older non-turbo 900 -- you need one from a 91-93 non-turbo or certain 9000s -- see eEuroparts.com for a list of cars that have that sensor. Yours is a potentiometer type and the other models had 3-position switches.

If it is a potentiometer then hook your DMM to it and check resistance for any dead spots, Do this both with the pedal and by hand with the linkage dissconnected.

ok.. now to sound stupid (and will most likely lead to a "doh!" moment)... DMM?

Digital MultiMeter

+1 on bad TPS. Probably has worn out in one particular spot.

doh! I knew it was coming.... what's funny. I have my father's MM from when he was in the navy... it's about 40 some years old

Per classes I took on this, an analog meter may not always provide proper loading, meaning you burn up things like a TPS because the resistance through the meter is too low. Digital is the only way to fly if you don't have an o-scope around.

harbor freight has some cheap ones.. guess that is where I am going tomarrow

Different horses for different courses. The problem with DMM is the display latency, usually about 1/2 second, sometime longer. Noise or intermittent connections on a potentiometer won't really show up, whereas an analog meter is 100% realtime, and meter tip will 'wiggle'. An O-scope is preferred over any meter. A Simpson 260 analog meter uses 50 microamps on the resistance scale (similar to what most DMM use BTW) No automotive grade component will be damaged by that amount of current. TPS in particular must be very rugged, the automotive electrical environment is hellish.

Carter

I was tinkering with the TPS today before going out. I pulled the connection and was (not really) surprised to find the middle pin was green with corrosion. A little connection cleaner and she runs a lot better.

The Simpson 260 is a great meter but I don't think your average home mechanical would have one. I don't have one handy but I believe the resistance through your average $15 or less Rat Shack analog multimeter is way less than the vaunted 260. And I do agree with display lag issues with a DMM.

If I were serious about working on cars I'd probably get an older HP o-scope, a good cover and a cart to wheel it around on. But that's not how I make my money.