996 Plugs 3 & 5 fouling out

I am new to the forum... Hoping someone could shine some light on this strange issue. 2001 carrera . 911 I recently pulled the engine and sent the heads to be rebuilt...There was low compression and leak tests were showing intake values not sealing. I used "Jones autowerks " with a good reputation. I since have reinstalled the heads and engine but I am getting excessive fuel on Cylinders 3 & 5. The car will start up but lope like these two cylinders are not functioning. The spark plugs get fouled out (black) with about 5 minutes of running. I have changed the plugs, coils, injectors. A present, the fault codes are P0300, P0301, P0303, & P0306. ( cylinder misfire) Note: these do not correspond with the fouled plugs I pulled out? Is there some mismatch with the DME... ? Good spark at each plug. I have verified wiring from DME to each injector and coil.. Fule pressure at the rail --- 55 psi The compression on all cylinders are good 185-195... I have verified the cam timing several times... I have measured temperature of the exhaust pipes from all ports and have found 3 & 5 much hotter than the others ( + 90 degrees) pointing to excess gas burning in the headers. I have a durametric monitor but would need what to measure to gather more data on this. Once revving above idle ( 2k) it seems ok but I have not taken it out until I can find some reason for this excess gas issue and loping idle. Any suggestions on finding or getting closer to an answer to this is greatly appreciated

If the 55 PSIG was measured during idle, that's too high. The spec is 48 PSIG +/- 3 PSIG @ idle. You can connect a hand vacuum pump to the fuel pressure regulator vacuum port (and plug the vacuum line) and check if the pressure is lowered from 55 PSIG to 48 PSIG when vacuum is applied. Also check the RKAT (or TRA) of both banks in Durametric, which are the fuel trims during idle. Check if they indicate reducing fuel. Also check the pre-cat O2 sensors to see if they swing between 0.2 to 0.7v about once a second at idle.

The RKAT = 0 RFA = .99 for each bank ... at idle 830 rpm I did check the full pressure at idle and it did drop to 48 Does this provide any more direction to the issue. I will check the pre-cat 02 on the next run... Is there something common for these two cylinders react this way... Will vacuum or complete loss of vacuum provide an over rich full mixture like this?

Move the coils and see the full the missfiring follows it.

This might be too simple. Are you sure that the coil connector bales are snapped over the coil teeth on the two cylinders?

Immo, the OP said he has already replaced all coils, plugs, and injectors, i think. Your idle speed sound very high if the engine is warmed up. The spec idle speed (a/c off) is ~680rpm warmed up. It sounds like vacuum leak, purge valve leak, or idle stabilizer acting up since you said it feels ok if revved up.

Thanks all for the replies... I did change all coils... with good connections... Has there ever been issues with the voltage connectors to the injectors... ( not connectiong securely) Thoughts are only when the longer pulses are performs do all injectors fire.. Is there anyway to verify proper cam timing with spark and full injection. ? I have used a noid light on the injector, a compression gauge on the cylinder, and watched the spark on the external coil/plug.. while cranking ... The light lights up late... I assume the delay may be to the noid light excitement.

How many miles on the engine? Hopefully you measured the cylinders for wear & roundness while the heads were off. How did the combustion chambers look on the suspect cylinders? Have you tried to clean the injecters? Every set of injecters I have had tested off the car reveals at least 1 that leaks.

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Any time the injectors are pardon the expression, messed with, there's always a chance of the injector or its connection to the fuel line or harness, again pardon the expression, getting messed up. So, you have to confirm all injectors are working properly. Generally this is done by with the engine idling disabling each cylinder's injector and noting the loss of idle speed. The cylinder (or cylinders) which result in the least amount of idle speed drop off when disabled are the suspect cylinders and injectors. The idle speed drop off is less because the cylinders are contributing less to the engine's overall output. Keep in mind that if both 3 and 5 are "hot" this can be because they are bearing the brunt of the work. The problem might lie with the other cylinders which are weak for some reason. The DME will add fuel to try to get the O2 sensor readings where it wants them and this can result in misfires (rich misfires) at the healthy cylinders. Also, be sure you have connected the VarioCam (Plus?) solenoid/actuator to the wiring harness. If one or both of these is not connected properly this can also lead to misfires as the DME tries to change the fueling to to get the sensor readings it expects but the cylinders go into misfire mode before the sensors come around.

The over working 3&5 cylinders seem to match the idea of really functioning and the misfires on the non fouled cylinders are really failing. 1.2.6 I have had the injectors cleaned but still need to verify these are good. I have pulled the intakes off and hoping to use a remote camera probe to view the spray pattern during the cranking phase. The plugs are also removed. Does this seem feasible. Is there any way to tighten the wire connections to the injectors. These seem very loose... Again thanks for all your help.

The problem looks to be several bad injectors. The ones that I had sent out to be cleaned!. Checking with the remote video camera down the intake showed only injectors 3&5 were firing- spraying. They were getting fouled by the DME sending more fuel because of the low O2 reading ( only two cylinders were firing) from the cat sensor. I pulled them out and could not get the relay to activate using a 5volt supply. Where the good ones would. I could only get bad ones to fire using a 12 volt supply. I assume the cleaning company was using 9-12 volts for the test. This higher voltage could have messed up the injectors as well since the DME uses 5-6 volts to activate. I am buying new injectors. Any recomedation on where to get these. Pelican has them way to high $230 each. Several web searches has also shown out of stock response on OEM injectors. The stock injector uses 250CC volume metric. There are some having higher volume 400-750cc . I was wondering to buy the higher volume ones with the expectation on short term add-ons to increase HP. How Would the 400cc effect the stock 996 ( and DME) prior to the upgrade of more performance add ons?

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If you have id'd a number of injectors as bad I'd be tempted -- were it my car -- to just replace all 6 with new. With injectors, as with plugs and O2 sensors, the idea is to keep things the same as best as one can for all cylinders and cylinder banks. To have a couple of cylinders with new injectors and the rest with old injectors could have some cylinders operating more (or less) efficiently than other cylinders. The expense of new injectors is high to be sure but they play a vital role in how the engine runs and with injectors that is not where one wants to cut corners. If you add higher volume injectors the DME will have to adjust the injector active pulse width - make it shorter -- in order avoid injecting to much fuel. What can happen is the DME will reach the limit of how much it can shorten the injector pulse width (around 10% is the threshold) and if this threshold is exceeded set one or more error codes and turn on the CEL. Unless you have a real new for more fuel to the point the DME will stay within its acceptable range of injector pulse widths or are using a custom flash which modifies the fueling map the DME uses and allows it more leeway (or puts in a starting bias) so the DME doesn't have to go too far out to bring the fueling into spec my advice is to use the stock injectors.

How about a good set of used injectors?

Parts Geek $60 to $ 86 each .

The 6 new injectors fixed this issue.. thank you for all the help... i contacted the company that cleaned my old ones and returned my cash...