This 928 had been sitting for the better part of a decade before the PO tried to bring it back to life. I fixed the LH system and MAF was just rebuilt. Starts and idles fine for a few minutes then this noise begins to start. Any suggestions?

 

Being an S3 ('86 going by the hose clamps) there could be a multitude of causes.

First off, the air cleaner element is upside-down






Bridge B->C (idle set mode) at the diag plug at the front of the engine with a wire and see what happens to the idle.

Test port info - https://rennlist.com/forums/928-foru...-85-86-us.html

LH/EZF pinouts - https://rennlist.com/forums/928-foru...2-pinouts.html

 

Sorry it is a 86.5 and I’ll fix the air filter. I’m in the SF Bay Area as well.

I will try bridging the test port tomorrow. What should I expect to happen?


A few more notes. New o2 sensor, light intake refresh with injectors ultrasonically cleaned, all new injector harnesses and intake runner gaskets.

Today it started right up, idled reasonably for a few minutes and I took it around the block. Throttle response was extremely poor. When I pulled back into driveway and put into park, the surging started happening. I turned it off, let it sit for about 5 minutes and started it back up, normal idle for a few minutes then back to surging.

Could it be a faulty temp 2 sensor?

Thanks for the help

 

What rpm range is the idle fluctuating...is it sitting around the correct idle of around 700 or so....

 

Could be Temp II (I'm not sure of the test specs) but check your ISV, Idle Stablization Valve operation and it's electrical connections. There is a plug connection at the ISV, and another plug "junction" closer to front of the engine, near the test port. The plugs at that junction are often cracked and bad. (There is a sort of 'extension cord' between the ISV and that junction at the front. If yours is bad, Roger sells reproductions made by Sean.)
-Jason

 

So if I cold start, it starts up and idles around 7-800. Then when it gets closer to operating temperature, the surging begins. It surges around 1400-1700 rpm and the torque makes the car rock side to side.

Since the temp 2 is pretty cheap, I’ll try that, then I will start looking into the ISV, which I have a sneaking suspicion is probably the culprit. I guess I’ll take it apart and see what’s what. I believe the connection is the same as injectors, but I’ll double check.

thanks for the help!

 

Replace the temp 2 sensor connector and
temp 2 sensor

Get another Known good MAF and put it in without disconnecting the battery see how it runs

 

I ordered new harness and Temp 2 sensor, hopefully that will address this surge issue and I can get this road worthy.

My MAF was done at injection labs so I hope it’s good to go.

The throttle response is also very sluggish, so I’ll get with Roger and try to sort that out.

Thanks!

 

No change when I bridged these ports.

I have replaced the temp II sensor and harness and the spark plugs, but the same issue starts happening about 2 minutes into idle. It is idling higher than I thought, closer to 1200, but this sound is strange. I pulled the air filter off to try and locate where it's coming from and it sounds like the throttle body. I took another video of the sound coming from top of the MAF. It seems stable idling outside of that sound, comes to operating temp and holds, but there is little to no throttle response when I move the throttle which leads to think the TPS/Throttle body may be the issue. I took out the idle control valve, cleaned it and it seems to spin easily. The pulsing sound is what I can't seem to figure out, the vacuum is good, but I don't see how even if there was a small leak, the pulsing sound would be related. Especially since there is little throttle response.

Any suggestions would be very helpful, thanks

This is a S3, the MAF was rebuilt by Injection Labs, New LH from Electronik Repair who also checked the EZF. I have replaced the spark plugs, injection harness(all), ultrasonically cleaned the injectors and performed a light intake refresh.

 

How many miles?

The S3 ISV/ICV/IAC is a wear item. If it is original it most likely needs replacement.

https://rennlist.com/forums/928-foru...alve-wear.html

Worst case the LH idle power transistor(s) can be damaged by a worn ISV.

 

Did you check the ISV? It's a bitch to get out but with small tools and patience you can take it out without pulling the whole top. Take it out and check it with a 9 volt battery, it should move smoothly and easily in both directions when power is applied to one of the sides and ground. Note it has brushes inside of it and they do wear down.

If it sticks by all means try to clean it but it must be smooth even if you put a finger down the barrel and press a bit on the moving part. If it shudders then it has dead spots on the commutator.

Do not buy a $30 replacement. They are 100% guaranteed to be cheap chinese ****. Spend the money and buy a good one.

I went through all of this. Other odd air sucking spots include the intake rubber gaskets being folded over (oddly enough), cracked, and loose. Consider just replacing the bunch of them. See if your plenums literally *move* when you floor it.

 

Thanks, I think I have some kind of leak, but I’ll test the ISV with a 9v too. I’m going to pull the top off again and double check all my vacuum lines and hoses. When I took everything apart the first time, all the rubber hoses and vacuum seemed in very good condition, and while I put every hose back where it came from, I’m wondering whether something may have been connected wrong to start from. I’m going to double check all connections and seals then figure out what’s next.

 

I tested the ISV with a 12v battery, it opens and closes firmly when I reverse polarity. I assume it’s only suppose to rotate partially, maybe 1/4 turn or so. It’s a Bosch unit with part number, not a Porsche so I’m assuming it’s aftermarket.

With the runners off. I started testing the vacuum lines at different points and have some questions from the above picture. The fuel regulator and dampers hold vacuum when applied, as does the line to the ECU. Part 15 doesn’t hold vacuum but I think that’s ok since it looks like a vent (correct?)

13- Thermo Switch - how to test this? should this hold any vacuum?

14- Air Bleed Valve- should this hold any vacuum? This was aftermarket with a VW part number, what’s the correct Porsche part #?

I did find a broken brake booster check valve, but I doubt that’s the reason for my surging since those lines seem to be related more to the HVAC system.

I’m trying to pinpoint whether I have a vacuum leak, and have read the tech guides on the vacuum system but apart from testing individual pieces, it’s unclear which parts should hold a constant vacuum and others that apparently don’t. Any help here would be welcomed. The vacuum pressure when idling is 17-18 in Hg, that dips to 14 during the surge.

thanks for any help!