Art20c

87 5.0 s4
What are the simptoms of bad o2 sensor or tps? Got problem which is hard to explain or understand. Some times (being going on for year) idle starts acting up by going down to almost 0 and then bounces back and down on and on. If I hit gas when it does that, feels like its out of gas, you push the pedal and engine goes "booo" and either dies or slowly rases rpm, I pump pedal twice and idle rises, but still not all 8, some times, even pumping pedal dont help, it just stalls, I crank it , it starts for a second or two ok, then dies down. when it idling up and down, some times if I put pedal to the floor, it just feels like one or two cylinders firing with rpm of about 0, and cat converter is red(but good exhoust pressure at the tail pipe) if i continue to hold gas down. As soon as all that clears(may take second or half an hour) engine runs fine, cat cools down and car runs fine. Even when driving on highway, I feel that engine is missing, and then over sudden, its gone, its firing on all 8.
when its bad, no smoke out of tail pipe though. Just passed NY state inspection with readings of almost zero.
Could it be water in tank or O2 or TPS?
P.S. I had chips replaced with Autotority chips 1.5 years ago

joejoe

So the problem started around the time new chips installed? Maybe swap back to original and see if problem dissappears.

Garth S

Because there appears to be an erratic nature to this issue, I would try to find a buddy with a good running S4 that you can swap your MAF and LH module into: if his car continues to run well, no damage done, and your more $$$ components are OK.
Until you line that up, I would check everything in the ignition circuit: primary and secondary wires to each coil, the condition and harness to the two control modules on left front ahead of the rad, plugs & wires, and dist. caps and rotors.
It does appear that you are losing 4 cylinders at a time - which is generally a coil or control module: the result is running rich ( half of the fuel is unburned in the cylinders) .... which then frys away in the cat .... approaching nuclear meltdown!
I would hope that doing the ignition stuff will solve this problem - were it my car, I'd never drive it if the cat started a sunset red glow.

Art20c

I would say 6 month after chips were installed
Also it doesnt seem that 4 cyl are missing, its absolutely erratic it might be 1 or 7, depends on i dodnt know what. Just now, drove it to work, (about mile), idle was acting up on intersections, but after few gas pedal pumps car was able to take off and run ok until next stop. Got to work though. Shut it down, tried to start again, it starts and immediatly stalls, even pumping gas pedal not helping, it just feels that not enough gas, but when I hold pedal down engine just bearely runing on 1 cyl, cat started to become red. Locked the car and went to work. See what happens after. I will change chips back. Also, ign modules on pass side afront of a/c condenser?

Art20c

Anybody knows good honest 928 diagnostic in Greater New York area to have them examine and fix this problem? Its driving me crazy to the point I want to wreck this car. It lets me down every time I need it tne most. Also, how porsche dealer charges for some tricky problem like that with diagnostics involved. Are they gonna change everything and hit me with gigantic bill, or they will pinpoint the problem?

NoVector

Art20C - You must be my twin! I'm trying to figure out the same symptoms. Runs great when cold, but after about 15-20 minutes, starts struggling to keep running. I recently replaced the head gasket, so God knows what I screwed up along the way. To help in the diagnosis, I bought a timing light and put it on each plug wire. When cold, each wire has a constant steady pulse. But, after 15-20 minutes, the pulse on all wires is erratic. Not good as it's bound to be spitting out unburnt gas.

I'm like you, I think it's an oxygen sensor issue--although my car doesn't have one. The PO put headers and a y-pipe with no cat on the exhaust with no o2 sensor. But I do recall reading on the list (I think) that the o2 sensor isn't read 'til after the car is warm. Which could be aprox 15-20 minutes.

FWIW - I've replaced the temp time sensor, temp II sensor, green wire, distributor cap and rotor, plug wires and plugs. Next is the 4 and 6 ohm resistors. I believe that's what Garth was talking about. They look like two white tampons with wires on both ends on the driver's side near the radiator. I also ordered a new coil... If none of that fixes it, I'll be tapping the y-pipe and inserting an o2 sensor.

Good luck my brother and PLEASE post your result when you solve this! I'll do the same.

Bruce

Art20c

NoVector-I did check those ignition modules on driver side panel, where hood lock is, by disconnecting one at the time. With one or other dissconnected, engine runs on 4 cyl, and you can feel it, but keep doing the same, running rough up and down. I mean, you can feel that only 4 is firing, which is not my case, cuz when I connect both back, it feels right away that 4 more are trying to run. With disconnected harness from MAF, idle is smoth and steady at low rpm. Is it MAF? or TPS? I will keep you posted.

TRUESCOTSMAN

https://rennlist.com/forums/928-forum/309375-wasn-t-idle-problem-looks-like-fuel-injectors.html


Check this post

Tony

The TPS is an easy test with a Ohm meter.
Check the 2nd page on this thread...i posted a few pages from the LH and EZK test manual.
https://rennlist.com/forums/showthre...5&page=2&pp=15

O2 sesnor is cheap but a PITA t replace on a stock exhasut set up.

Id lean towards a bad MAF though.
Where in NYC are you?

Mattman928

Probably not the O2 Sensor, But that is easy to test. If you have access to an Air/Fuel meter I would install that (I have found to be a wnderful troubleshooting tool). If not, do the following:
- Disconnect the Battery
- Unplug the O2 Sensr (Plug is on passenger footwell in front of fuse panel
- Wiat 30 seconds then reconnect the battery w/o the O2 sensor
- Start the car. If the car runs well (maybe a little rich, but consistent) then it may be the O2 or the LH brain, or the wires in between.

In my opinion it sounds like a vacuum problem on the fuel pressure regulator or the dampers.

Another option could be the ISV, if the problem only occurs around idle. There is a good writeup somewhere on the forum for testing the ISV by spraying WD 40 down the vacuum line that feeds the ISV.

For the Temp II sensor, measure the resisitance between a gound point and either spade on the sensor, if it works there, also check on the LH connector:
• 32 deg F, resistance should be 4400 - 6800 ohms.
• 59 deg F to 86 deg F, resistance should be 1400 - 3600 ohms.
• 104 deg F, resistance should be 900 - 1300 ohms.
• 140 deg F, resistance should be 480 - 720 ohms.
• 176 deg F, resistance should be 250 - 390 ohms.

For testing the WOT Rich has a great image of the LH connector at his website (http://www.electronikrepair.com/wst_page7.html), Please note that the Pin-out is reversed if your car is a 1990 or newer.

Enjoy

John Speake

You could update your '87 LH and EZK to diagnostic versions by swapping the EPROMs to the '88 ones. Then you can check out any stored fault codes.

Jim R.

Intermittent running on less than 8 cyl. and glowing cat is not too good or safe. Does one cat glow or both? Have you checked the coil wires and grounds? If any are loose or corroded that would be a simple fix. Also, how old are they? If the O2 was bad or the MAF was bad I doubt you would have passed NYS emissions testing, but I am not the diagnostic expert here. I would start with coil wires and grounds cleaning--cheap and easy.

I would stay far away from a dealer for diagnostic work on a 928. If I was stuck and could not figure it out, I would probably take it to Musante and David LLoyd in CT. Where in NY are you?

Jim

John Speake

Agreed that this problem sounds very much like an igntion system issue.

Diagnostics would not help with type of problem.

Art20c

I am in Brooklyn.

Art20c

Both cats are red.
I am in Brooklyn