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sighting's Avatar
 
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Question Rough running smoothish idle

I have searched high and low and was hoping to get some help here. (everyone I found was for MAF and ECU cars

I recently had a change to buy a 1979 928 US manual and jumped on it.
It ran a bit rough but the body was perfectly straight with no rust and a manual!!

Fast forward a few days now and I am stumped at what could cause it to run so incredibly rough.
It's a constants rough not sporadic.
Every thing I replaced below made no difference to how rough it runs (maybe cleaned up the idle a bit)

Current State:
* Starts up Immediately - does not matter if it is a cold or warm start.
* Idles perfectly (not sure on specific RPM as the cluster is a bit jenky)
* As soon as you rev it it starts to run rough.
* It's drivable but it feels like its running on 4 cylinders.
* Temperature never exceeds 50% even at driving for an hour
* Oil Pressure stays at 5 ( I have seen it at 3 so I assume the gauges work
* It feels like it smooths out at higher RPM but it has no power then
* No smile out of the tailpipe
* No backfires or pops

Things I have done:
* Replaced Spark Plugs (Old spark plugs all looked exactly the same - Normal with a lighter color - maybe a bit lean)
* Replaced Rotor
* Replaced Distributor Cap
* Replaced Plug Leads
* Replaced every single Vacuum line I Could find
* Fresh Tank of gas (91)
* Seafood and Injector cleaner directly and into tank
* Replaced Air filter
* Replaced oil and oil filter
* Replaced all blown fuses

Things I am waiting to be deliver to be installed:
* Fuel Filter
* Looking for my old spare 944 Turbo Coil to try it (will check to see if its compatible)
* Visual inspection is not giving me any clues.

I really hope someone here can help direct me in the right direction as I can feel hints of how the car wants to run.
I am open to all suggestions tia


Last edited by sighting; 02-11-2020 at 08:17 AM..
Old 01-22-2020, 11:17 PM
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Dirty fuel injectors possibly. Clogged fuel filter. How is the compression? Do a compression test. Vacuum leak test? Sounds like ignition or fuel issue. When did it last run normally? I have a later model, maybe someone with your year car can be more helpful.
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1986 928S
32 valve engine
All stock, automatic, 539 Weissgold Metallic, 70K original miles, Hankook Ventus 2 tires.
Previously owned: 67 Vette, 427 L88 Stingray, 74 De Tomaso Pantera L. Latest addition: 2000 BMW Z3 Roadster
Old 01-24-2020, 06:01 AM
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I am kinda chasing a similar issue... 82S Euro that was running great and as of last Sunday started running rough. Car shakes at idle and when revved up seems to be running on 7 cylinders or less. Gas is maybe a month old.
Old 01-24-2020, 09:02 PM
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Also check O2 and Temp sensors, timing belt.
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1986 928S
32 valve engine
All stock, automatic, 539 Weissgold Metallic, 70K original miles, Hankook Ventus 2 tires.
Previously owned: 67 Vette, 427 L88 Stingray, 74 De Tomaso Pantera L. Latest addition: 2000 BMW Z3 Roadster
Old 01-25-2020, 04:49 AM
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See if all the timing marks line up, it's possible the belt jumped a tooth.
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George in Indiana
928 Weissach #153
Cayenne S
Old 01-28-2020, 05:32 AM
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Start cheap and easy - clean all grounds - nice and shiny. Especially the one from the lower engine block to the engine bay on passenger side, and the one in the trunk from the battery to the chassis. Also, your car is a CIS K-Jet, and very susceptible to pressures, which will affect running conditions. Test the pressures per the WSM (work shop manual). If you don't have the WSM, get it on disk from Roger at 928 R Us. You'll need pressure gauges, or take it to someone who knows the CIS system. The injectors are manual, and not serviceable. There are some guys who can clean them though. However, if you have steady idle, not likely a clogged or dirty injector issue - it would be rough under all conditions. I'm thinking grounds and/or fuel pressures in the CIS.

Oh yeah, and remove the seafood from the tank - that could be causing a problem (j/k)
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Ed
78 928 5 spd "Blumaxx" (Minerva Blue; featured in issue #100 Hemmings Sports & Exotics)
05 997 (Meteor Metallic Gray over terra cotta)
06 Mustang GT (Legend Lime)
Old 01-29-2020, 10:09 AM
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Quote:
Originally Posted by Linderpat View Post
Start cheap and easy - clean all grounds - nice and shiny. Especially the one from the lower engine block to the engine bay on passenger side, and the one in the trunk from the battery to the chassis. Also, your car is a CIS K-Jet, and very susceptible to pressures, which will affect running conditions. Test the pressures per the WSM (work shop manual). If you don't have the WSM, get it on disk from Roger at 928 R Us. You'll need pressure gauges, or take it to someone who knows the CIS system. The injectors are manual, and not serviceable. There are some guys who can clean them though. However, if you have steady idle, not likely a clogged or dirty injector issue - it would be rough under all conditions. I'm thinking grounds and/or fuel pressures in the CIS.

Oh yeah, and remove the seafood from the tank - that could be causing a problem (j/k)
By all means clean all ground points and also fuse panel, fuse holders. Braided battery ground strap may look good on the outside but bad on the inside.
__________________
1986 928S
32 valve engine
All stock, automatic, 539 Weissgold Metallic, 70K original miles, Hankook Ventus 2 tires.
Previously owned: 67 Vette, 427 L88 Stingray, 74 De Tomaso Pantera L. Latest addition: 2000 BMW Z3 Roadster
Old 01-31-2020, 06:17 AM
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Thank you all for the replies,

Update

More Items Replaced:
Fuel filter
Ignition coil
Green Wire
Soaked and Ultra sonic cleaned out the injectors as much s possible.
Completely rebuilt distributor (took it apart and cleaned and tested it - mechanical advanced springs were off and the vacuum was stuck)
Fuel distributor rebuild kit
- Broke fuel line to Cold Start injector!!!

Bought a Fuel pressure tester and with the pressure tester connected to the WUR from a cold start:
1. It started low then build up slowly to 52ps and stayed steady at 52psi
2. The WUR did its job on a cold start and slowly brought the RPM's down to 800rpm around the 2min mark.
3. When I closed the valve to bypass the WUR the pressure moved to 73 PSI.

Double triple checked the timing and made sure belt did not jump on the cam timing.

When I put it all together it sounded exactly the same apart from it running really rich now.
That means I probably fixed fuel pressure and / or vacuum leaks..

But the rough (running is still there)

Last edited by sighting; 02-11-2020 at 08:28 AM..
Old 02-11-2020, 08:24 AM
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More Updates:

So I decided to take it out all again.

I gave the engine bay a very thorough cleaning and removed the intake runners and main CIS unit.
I noticed that the CIS unit was completely loose in the body as the 4 rubber mounts were sheered off.

Took CIS off and cleaned out the unit completely.

Installed brand new Bosh injectors and Seals

While I was cleaning I noticed a little crack in the front 14pin harness with a brittle cable.

Needless to say, too turned out that it was not little and in fact was a mayor short at some time and the complete front harness was shot.

The more I peeled back the sleeving the worse it got. The main starter/alternator wire shorted with other wires around it and cooked them all. I have no idea how this car even started our ran>??

So I have been rebuilding that complete cable for the past few days and hope to be done today with it.

In the process, I am repacking a few more vacuum /water lines.

I now have 2 questions I hope someone can help with.

1. Does anyone have a pinout for pin14 for me?
- All the wires pretty much are the same color - and are badly burned or missing sleeves.
- I numbered as good as I could but I just want to be sure.

2. I redid the vacuum lines according to the (928 Dimensions and tolerances 1978-80 book)
- Seems like the PO switch some lines
- I have a vacuum line that I cant seem to find where it goes to. It is on the passenger side and comes from the 5 splitter under the main intake and the goes into the passenger fender (might be fuel vapor related?) see picture below

I am quite optimistic about sorting out the rough running today

Please remember my car is a Manual 1979 US version.

Appreciate the help./
Thank you

Last edited by sighting; 02-11-2020 at 09:20 AM..
Old 02-11-2020, 08:41 AM
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Old 02-11-2020, 09:22 AM
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Sigh, think I made it worse. its running incredibly rich now and I am unable to ever get it to IDLE correctly. Back to the drawing board I guess.
Old 02-12-2020, 12:29 PM
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Mrmerlin on here is very knowledgeable. Look up his garage and send him a note. My car is an 86S so not the same as yours. You can also get on Rennlist Porsche site, all kinds of info and help there. Good luck and keep us posted.

Rudy
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1986 928S
32 valve engine
All stock, automatic, 539 Weissgold Metallic, 70K original miles, Hankook Ventus 2 tires.
Previously owned: 67 Vette, 427 L88 Stingray, 74 De Tomaso Pantera L. Latest addition: 2000 BMW Z3 Roadster
Old 02-13-2020, 06:28 AM
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Thank you will do.

So connected a smoke / leak tester and tightened up a hose here and there.

But the system is leak free as far as I can tell.
I have a CO2 meter now as well and played around with it this morning / afternoon.

I got it running and semi idling but had to unscrew the front screw allot as in 90% out and also backup the A/F.

So it seems its now a combination of setting both those in the correct manner.
I am trying to find a guide that I can use to set it from SCRATCH as the PO or previous shop clearly change the ratio instead of fixing the low fuel pressure and multiple air leaks it had.

Does anyone know if there is a base amount fo turns in each of them to get me at least in the ballpark to get it to idle correctly.

PS. fire department came by today because someone thought things were on fire
It was burning off the excess injector cleaner I had in there to clean up the valves.

Looking at the Manual I followed the instructions for "Adjusting the Idle and CO" but again it feels like my base is way off.

When I adjust the CO setting leaner the idle shoots up to over 2k rpm I then have to screw the "idle controls screw in most of the way to get the idle to drop"

When I get it to drop it does it fairly quickly and then stalls. It would refuse to restart until I back the Idle controls screw out allot again - OR if I make the setting richer.

Can anyone locally help or recommend someone here in the Bay Area (California) that does not want to charge me 4 hours minimum nat $175 per hour to look into this?

TIA..
Old 02-17-2020, 02:05 PM
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Quote:
Originally Posted by sighting View Post
Thank you will do.

So connected a smoke / leak tester and tightened up a hose here and there.

But the system is leak free as far as I can tell.
I have a CO2 meter now as well and played around with it this morning / afternoon.

I got it running and semi idling but had to unscrew the front screw allot as in 90% out and also backup the A/F.

So it seems its now a combination of setting both those in the correct manner.
I am trying to find a guide that I can use to set it from SCRATCH as the PO or previous shop clearly change the ratio instead of fixing the low fuel pressure and multiple air leaks it had.

Does anyone know if there is a base amount fo turns in each of them to get me at least in the ballpark to get it to idle correctly.

PS. fire department came by today because someone thought things were on fire
It was burning off the excess injector cleaner I had in there to clean up the valves.

Looking at the Manual I followed the instructions for "Adjusting the Idle and CO" but again it feels like my base is way off.

When I adjust the CO setting leaner the idle shoots up to over 2k rpm I then have to screw the "idle controls screw in most of the way to get the idle to drop"

When I get it to drop it does it fairly quickly and then stalls. It would refuse to restart until I back the Idle controls screw out allot again - OR if I make the setting richer.

Can anyone locally help or recommend someone here in the Bay Area (California) that does not want to charge me 4 hours minimum nat $175 per hour to look into this?

TIA..
Just to help. Put the model and year in your signature, so people can see what car you have.
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1986 928S
32 valve engine
All stock, automatic, 539 Weissgold Metallic, 70K original miles, Hankook Ventus 2 tires.
Previously owned: 67 Vette, 427 L88 Stingray, 74 De Tomaso Pantera L. Latest addition: 2000 BMW Z3 Roadster
Old 02-17-2020, 02:33 PM
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@harborman I updated my signature. thnx

Quick Update:

I added my signature.Following the workshop manual procedures I got it to 2.4 on the CO2 meter but had to turn the front "Idle controls" screw in alllll the way down. Which does not sound right. took it out for a drive and it feels the same as when I started


Last edited by sighting; 02-17-2020 at 03:37 PM.. Reason: Updates
Old 02-17-2020, 03:33 PM
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