Carpokes Porsche Forum

Tom wrote: Mon Oct 16, 2023 9:56 am Let's see....

1. Yes, an exhaust leak can throw off the emissions. An exhaust leak at the header let's exhaust gas out, and fresh air in. When fresh air trickles into the exhaust, it will cause the oxygen sensor to think the car is lean, and the DME will try to add more fuel and the car will run rich. High CO readings generally mean the car is running rich. I assume your car has an 02 sensor, right? Either way, when exhaust leaks into the engine bay, some of it will get sucked into the HVAC inlets, and then into your lungs.

Yes I have a completely new Bosch O2 sensor installed. I actually have a new theory for my whole crossover problem. I always wondered why I didn't notice the car smoking during my inspection when I purchased the car back in November 2022. It got me thinking and today I formulated this theory:

The car is leaking oil that drips on the crossover pipe (this is a fact). The PO still drove the car up until I purchased it (he said so himself). This means that at the time of purchase, the crossover was oil free! This means, no noticeable smoke since the oil spill was burned off.

When I started the car in May 2023, after a timing belt and waterpump change, the car immediately developed the smoke, clearly coming from the crossover. I thought it was a leak, but it may have been oil burning off as the car had been sitting for months, and thus had the time to accumulate the oil on top of the pipe.

I may have proven this theory already, since when I 'fixed' the pipe with some exhaust paste a week later, the smoke was immediately less and after a few minutes practically gone. The oil must've burned off again, making it appear that my bandaid fix resolved the 'leaky pipe'.

After this fix, I went to the DMV and failed emissions testing. I thought my bandaid didn't work and that the pipe was too far gone. But it may be that the CO issue is caused by another part of the engine. I never did the soapy water test because it looked so obvious where the leak was.

I may have spend about 2K on crossover pipes I didn't need to!
However, seeing my old pipe, the new crossover definitely is an upgrade so.. yay?

2. You need to pull the headers off to change the gaskets (at least without cutting up the gaskets and compromising their integrity).

Thought so, unfortunately.

3. R&R'ing the headers is, on average, 6.38 on a scale of 1 to 10. However, it can be fairly easy if everything comes apart easily, or a nightmare if studs snap off in the head, etc.
Do you have a recommendation regarding a set for this job? Or perhaps a type of studs? Rennbay sells a kit that has a hex head stud which seems popular. Do you think it's good?

4. Heat can help at times. Propane torch works too, just be careful around the fuel lines. You don't necessarily need to get every last stud. I'd start with penetrating oil first, give up on the ones that don't budge, and break out the heat if needed once the easy ones come out.
Not sure I need to remove the cam tower for this job (opinions seem to vary) but using a torch would probably melt the gasket there, no?

5. I'd replace them with good used headers from Porsche. Welding stock headers is hard to do right, and aftermarket headers all tend to introduce new fitment and quality issues. I'd only do aftermarket headers (in the style of SFR headers) if I already had a high-output motor and was looking for incremental gains after doing all the common hot rod stuff.
I could look for some used headers but I'm scared of buying cracked ones. There are also a few types of headers for 944 turbos and I'm not sure if I should keep my 86 accordeon header(s) or get some others.

6. No, don't put goo on an exhaust leak. It won't work and the Porsche gods will curse your car.
Roger that!

7. White smoke/steam can be a bad head gasket, but if the car has been sitting for nearly a year, the crankcase has probably accumulate a lot of condensation/moisture. If, after you get the car running, you keep getting white smoke/steam, even after a long drive, then I'd start to worry. See the first few paragraphs of my HG replacement guide for that to look for... viewtopic.php?t=351
I'll check out your guide! I hope it's condensation, and frankly, would be kinda surprised it had a blown gasket... But I'll check for the signs this weekend. An oil change would tell me everything, right?

8. Oily black plugs after a few miles is concerning. Unless they are sopping wet, I'd get the car running and see what happens once the car has been run a while. After you burn off the one-year hibernation, if those plugs are still getting oily fast, then time to start running some mechanical tests.
I'll keep an eye on it.

9. If the header-to-crossover flanges are leaking, you might try loosening them all up an re-torqueing them. You can get away with a little extra torque (say 25% over spec) but if the headers leak with all the fasteners torqued to spec, something else isn't right. Since your headers were welded, I'm guessing they are the issue.
Thanks, I'll double check if there is smoke coming from them with the soapy water test.

Other stuff: very common to burn off oil and grease after replacing exhaust parts. Should go away after running the motor long enough. If it doesn't, then worry. Watch for oil dripping onto the crossover from a leaking rear main or oil pan onto other hot exhaust parts. That's a fire hazard. Hopefully, the white smoke is a year's worth of condensation, but time will tell. The black spitting from the tail pipe is just normal soot in the exhaust mixing with the moisture that's spitting out the tail pipe -- it's the byproduct of a potential problem, but not its own problem.

Belgian951 wrote: Mon Oct 16, 2023 1:53 pm

When I started the car in May 2023, after a timing belt and waterpump change, the car immediately developed the smoke, clearly coming from the crossover. I thought it was a leak, but it may have been oil burning off as the car had been sitting for months, and thus had the time to accumulate the oil on top of the pipe.

Possible, but in my experience, oil drips mostly after you drive the car and the oil is hot and thin. Once the car cools down, most leaks don't just keep dripping forever. If a leak were that bad, I'm thinking you'd know that by now. Maybe the seller just cleaned off the cross-over so it wouldn't smoke when he showed the car?? Or, like when my rear main was leaking on the cross-over, the oil leak could be hit or miss. I could go weeks with no smoke, then just a couple drops onto the crossover and it would smoke like crazy.

I may have proven this theory already, since when I 'fixed' the pipe with some exhaust paste a week later, the smoke was immediately less and after a few minutes practically gone. The oil must've burned off again, making it appear that my bandaid fix resolved the 'leaky pipe'.

Agreed. Also, exhaust leaks alone won't create thick oily smoke (unless that's also coming out the tailpipe). You are more likely to hear and smell an exhaust leak. If you are getting heavy oily smoke from an exhaust pipe, that typically means there is oil or grease on the outside of the pipe, rather than exhaust gasses leaking out.

After this fix, I went to the DMV and failed emissions testing. I thought my bandaid didn't work and that the pipe was too far gone. But it may be that the CO issue is caused by another part of the engine. I never did the soapy water test because it looked so obvious where the leak was.
I'd be inclined to pull the O2 sensor and see if it's covered in oil like those spark plugs. If it's bad, it can easily cause you to fail emissions. Also, if there is that much oil in the exhaust, then maybe the smoke you are seeing at the tailpipe is indeed oily exhaust smoke rather than condensation. Oil smoke is smelly and blue-ish and lingers in the air, whereas condensation is white and tends to evaporate rather than blowing down the street. Maybe post a video if unsure...

Do you have a recommendation regarding a set for this job? Or perhaps a type of studs? Rennbay sells a kit that has a hex head stud which seems popular. Do you think it's good?

The primary benefit is for 'next time' since those studs have an Allen socket to remove them. Particularly nice if you take the headers off a lot. If the studs end up sticking in the head, however, not sure an Allen socket would hold up.

Not sure I need to remove the cam tower for this job (opinions seem to vary) but using a torch would probably melt the gasket there, no? Not if you just put the flame on the stud...

I could look for some used headers but I'm scared of buying cracked ones. There are also a few types of headers for 944 turbos and I'm not sure if I should keep my 86 accordeon header(s) or get some others. Yes, that is the risk, unfortunately. On the other hand, I've yet to see aftermarket headers that are anywhere near as high-quality as the factory -- short of custom made tube headers that cost thousands and take months...


I'm pretty sure my rear main and oil pan are leaking... This car is a long-term project for sure! Good that the black spitting issue is not really an issue. Thank you for all the help! Ouch, sorry about that...but at least you'll really get to know the car.

Belgian951 wrote: Wed Oct 18, 2023 11:58 pm I already changed the O2 sensor because well why not. It was greyish if I remember. I can make a picture this Saturday. Replaced it with a brand new one. I'll also get a vid of the smoke. It's not blueish, just white. I'll also do an oil change and get a pic of the oil, I'll be sure to clean out the oil catch pan first.

I ordered a set of Rennbay studs and the exhaust headers gaskets, they'll arrive on Tuesday.

Would you say the Lindsey Racing headers are good? If I buy a new set, it's probably theirs. I'll keep an eye out for Porsche headers though. I know a store that sells new headers for cylinder 2 & 3 but not 1 & 4.

Tom wrote: Fri Oct 20, 2023 7:28 am The exhaust ports have a path to the tail pipe whether the waste gate is open or closed. The majority of it goes into the headers, through the cross-over, through the turbo, through the downpipe, through the cat, and out the tailpipe. To control boost, sometimes a portion of that exhaust leaks through an open waste gate, bypassing the turbo and downpipe, and goes straight to the cat. Either way, if you pressurize the exhaust via the tale pipe, the pressure will find every joint in the system.

I bought a smoke-making machine from Amazon when hunting for vacuum leaks on my old Mercedes. It eliminates all hunting -- every last leak sends up its own little smoke signal you just can't miss.

https://www.amazon.com/AutoLine-Pro-Aut ... 0aWM&psc=1