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Intermix Resurrection

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Good afterlunch, new member here with a 1999 996 Cab, 79k mi, manual trans, ocean blue metallic with saddle leather interior, Porsche Sport Design 18 inch wheels painted gold, K&N cold air kit,  muffler bypass, and lowered 3 inches all around. I love this car! Resurrecting the car from a cracked head/intermix and am encountering some strange behavior. Running Shout detergent through the cooling system to clean out any residual oil, had disassembled and flushed all the cooling system hoses/rads when I first took the car off the road in 6/18, so the current flush is more for the engine block. The strange behavior consists of extremely high oil pressure (4.5 to 5 bar) even at idle, ragged idle with plug fouling and, if I blip the throttle to overcome the ragged idle, the engine rpm"s hang up at 3-4k rpm whereupon I immediately shut off the engine. Consequently, I can't run the engine long enough to get it up to operating temperature so that I can then drain the detergent mix from the cooling system. I've had the car since 2014 and whenever I changed plugs, the old plugs had a light tan appearance. When I had the engine apart, I de-carboned both heads and all piston tops. Upon reassembly, I put 5 gallons of fresh gas in the tank. Now when I pull the spark plugs they are fouled and sooty, so today I installed new NGK platinums but the idle was still ragged and the rpm's would still hang up at 3-4k if I blipped the throttle. When I pull the new plugs tomorrow, I expect they will be fouled. Today I also drained the fresh Castrol 20W50 oil and replaced with fresh Castrol 10W30 thinking that the 50W oil was causing the high oil pressure. Even with the 30W oil, the pressure was still 4.5 to 5 bar. Prior to the cracked head I ran 40W and had 1.5 bar oil pressure at idle and 3 to 4 bar in normal driving, but had read that 50W was preferable which is why I tried it. I also removed the oil pressure sensor and could hear a slight rattle when I shook it (not that that means anything) and have read that when they fail electrically you can get either a zero reading or an extremely high reading, so perhaps that issue is a s simple as a failed sensor. When I had the engine out and apart, I took the sump cover off and cleaned out the oil pickup tube and the swirl pots. I only found one small piece of gasket sealer, everything was clean as a whistle which was gratifying. I just pray that I didn't reassemble the pickup tube/swirl pots incorrectly resulting in the oil pressure issue, although I did them one at a time so as not to mix anything up, and they are very simple and straightforward to reassemble. I replaced the AOS and the oil cooler, could those be the culprits for the rpm and oil pressure issues respectively? One final comment, the throttle cable appears to have shrunk by about one quarter inch in the 16 months the car was idle, no matter how I route it, the throttle butterfly bell crank is about one quarter inch short of the stop. Even routed it through the clutch slave coil and under the fuel injector rail feed lines, no luck but it will idle raggedly between 6-700 rpm. Apologies for the long post, look forward to your collective wisdom, thanks!

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In recently rebuilt my 99 996 engine for the same reason, cracked cylinder head and an intermix problem. Make sure that you have some free play on the throttle cable and I would either replace the oil pressure sending unit or check your oil pressure with a mechanical gauge. I’m currently using a 5W 40 oil and getting about 1.5 bar at idle when warmed up and between 3-4.5 bar depending on RPM elsewhere. 4-5 bar at idle when cold.



996 Forum - New to me 996 with coolant intermix issue - Hello there, I just acquired a 99 996 Cabriolet with a oil in the coolant and oil intermix issue. PO had replaced the oil cooler hoping that it would cure the issue but sadly it did not. Suspect a cracked head and wondered what other things should be looked at...



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Sir, thank you for your response and suggestions. I have already ordered a replacement sensor and hopefully that will resolve the oil pressure issue. Regarding the throttle cable, how would I achieve free play in the throttle cable? If the throttle body bell crank is contacting the stop causing the throttle body butterfly to be fully closed and the Idle Control Valve as the only air source, then the rear cable doesn't reach the front cable being one quarter inch too short at the plastic cable junction under the car below/behind the driver's seat even with the plastic cable junction fully adjusted in. When I pull the two cables together, the throttle body bell crank is one quarter inch short of the stop and the throttle body butterfly is slightly open. Is there any further cable adjustment available in the front cable from the gas pedal to the plastic junction? I did not disturb the front cable during disassembly/reassembly of the car, that's why I suspect the routing of the rear cable from the plastic junction, through the clutch slave coil, under the fuel injector rail and oil cooler feed lines, and over the top of the intake manifolds' crossover tubes as the source of this issue. Again, thanks for your input.   

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Try this link..


996 Forum - Throttle cable adjustment - Anyone ever have throttle cable break on their C-2, mines not broken but throttle plate seems to stay open slightly causing a high idle. If I manipulate the gas pedal the idle will go down to correct rpms. Not sure if the cable is sticking or if its in the throttle body...  

Hi Nordiques,


there is an orientation to the square plastic grommet at the throttle body. If not correct it may be your problem.


good luck,


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Thanks for that link, first time I've seen it even though I searched Rennlist several times regarding the throttle cable issue. Anyway, reading through that link gave me accouple of ideas to follow up on, as well as your suggestion regarding the orientation of the square plastic grommet. I've been putting it through the back of the throttle body with the longer locking tab up, so I'm going to try it at 180 degrees with the short locking tab up, and if that doesn't work I'll try 90 degrees and 270 degrees to see if any of those orientations gives me that last quarter inch. Regarding my ragged idle issue (getting misfire codes), today I removed the throttle body altogether, and then removed the Throttle Position Sensor and the Idle Air Control Valve with the intention of swapping the known good ones on my 1999 Boxster into the 996 to eliminate those 2 possibilities. Also going to swap over the MAF sensor as that is also a known good one. I pulled the MAF sensor from the 996 and cleaned it with CRC MAF cleaner, but a swap test with a known good one should also eliminate that possibility. Today I got the 996 to idle somewhat better, turns out the K&N cold air kit rubber boot between the throttle body and the cold air tube had loosened. So cranked down on the screw clamps and it idled better, but still threw misfire codes for bank one. The spark plugs are brand new and the coils are 2 years old, having been replaced on both cars in summer of 2017, and haven't gone even 5K miles. If all else fails, I'll swap the Boxster coils into the 996 in my ongoing process of elimination. Also, neglected to mention earlier that I changed out the fuel filter when I had the 996 engine out. Maye the fuel pressure regulator or fuel pump are wonky? Thanks for you response, I'll keep at it until the car runs the way its supposed to.

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4.5 - 5 bar when cold is not unusual. The engine doesn't have to be at "operating temperature" to drain the coolant/soap suds... It sounds like you're conflating several different issues and chasing your tail in circles. A good rule of thumb when troubleshooting is to deal with one issue at a time, until it's 1) identified, and 2) resolved. Don't move onto the next issue or replace parts that aren't identified as bad/broken as you'll just confuse yourself more, and possibly be throwing money away...

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dporto, thanks for  your input, yes, it does fell like I'm chasing my tail, but I think some of it has to do with how fragile these engines can be. The draining of the detergent: yes, I understand that it can be drained cold, but everything I have read about this detergent rinse process states that it should be drained at operating temperature for maximum effect. I am precluded from getting to operating temperature by the high oil pressure issue and the fact that I did disassemble, clean and reinstall the swirl pots and the oil pickup in the sump, and the apprehension that somehow I had botched that causing the high oil pressure. I've had this car for 6 years, am familiar with its oil pressure cycle from startup to idle to full operating temp, and can tell you for a fact that the oil pressure I am seeing is not normal. I've already ordered a new sensor, so hopefully that issue will resolve. Secondarily, ragged/erratic idle and sudden surge to 3-4k rpm keeps me from running the engine long enough to get to operating temp because, given the oil pressure issue, I don't want to blow out the main seals or cook the engine. Also, the rear throttle cable issue, which appears to be short one quarter inch, keeps the throttle butterfly slightly open which seriously compromises the car's ability to idle.  So there's a lot going on at once, but your advice is sound, I'm going to focus on one thing at a time. Thanks again!   

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So today I resolved three issues: the short throttle cable, the high oil pressure and the detergent rinse! The short throttle cable turned out to be the black plastic block at the end of the cable that snaps into the back of the throttle body. The block had separated from the throttle cable sheath, and when I removed the cable from the car this morning I saw what was going on, jammed the cable sheath metal tip into the block as far as it would go, reinstalled and, voila, the missing one quarter inch had reappeared! I was then able to adjust the butterfly and stop so that the butterfly closes without jamming, and the bellcrank rests on the stop! Now it idles at between 6-700 rpm, still somewhat ragged/erratic, and no more surging to 3-4K rpm. Plus, as an added bonus, the oil pressure sensor started working again! I had approximately 1.1 bar at idle and the oil pressure needle would rise and fall with the rpm's just like it used to, with 4.5 bar at 3-4k rpm. I ordered a new sensor which I will install anyway, but I'm gratified to know that my work on the oil pickup and swirl pots in the sump were not the cause of the high oil pressure. So, given that progress, I was able to get the car to operating temperature and then drain out the Shout detergent mix from the cooling system. There was not much if any oil residue in the drained detergent mix, but I'm going to do a 2nd cycle of Shout mix, then plain water, then install new coolant reservoir and low temp thermostat before finally filling with red antifreeze, which is what came out of the car originally. I did get a CEL and #2 bank #1 O2 sensor/cyls 4-5-6 misfire codes when the car was warmed up, which accounts for the ragged/erratic idle, so I am going to replace all 4 O2 sensors just to be sure. As they say in show business, that should be a wrap! Thanks to all whom took the time to consider and respond with their wisdom and suggestions, it is truly appreciated. I hope to pay it back by helping someone on this forum with their issues. 

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I believe the idle spec. is 750 - 780 rpm after initial high idle period (around 1100 rpm). If it's idling below that and roughly, I'd look for an air/vacuum leak. For your misfires you should check/replace (with emphasis on replace) your injectors. They're notorious for leaking - which can lead to serious bore scoring problems. Do not have them "re-built" as they will fail again in short order (there's no such thing as a true rebuild - they just clean them and change the O rings). Good luck

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dporto, thanks for the information, and for the suggestion regarding the injectors. I considered servicing the injectors while I had the engine out, clean them and replace O-rings and feed lines, but neglected to do so. I believe I am suffering from some long repressed trauma attributable to an injector service I did on a previous Jag XJS V-12 that was miserable and went sideways almost immediately. Suppose I was a little gun-shy, and the 996 was running beautifully right up until I got the low coolant warning light by which point there was already oil in the coolant and a crack in the # 1 head. Since I've owned the car (6 years/15k mi.) I've run a bottle of fuel injector cleaner thru the system at every oil change (3k mi.).To do the injectors now with the engine installed would be a horror. I inspected the bores as best I could w/o splitting the cases when I had the heads and sump off, and the bores looked surprisingly good. I have been getting mis-fire codes for bank #2, and inactive pre-cat o-2 sensor code for bank #2, so I am in the process of changing out all the O2 sensors and if that doesn't do the trick then on to the air/vacuum leak, and if the injectors have to come out they shall come out! Thanks again! 

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