what can cause higher hc at idlining
 

Ok I have 944 87 na 105kmiles trying to pass emissions!!I had some mods in car and at the first test time numbers where little high and didn't pass . so I took msd out, and replace original cat converter with brand new one, did a compression test what was good ! has compression 175all cylinders not leaks runs great, I took car back to test ! before test I drove car 30 minutes warn engine, everything was great but at end it fail one more time !!this second test point some weird results, when i accelerate the car it gave really low numbers almost zero in everything great!!! but when car is left at minimum rev without any gas pedal --- my hc number goes to 300 ,, I wondering what can cause that ? I know that mean two thing too much gas or oil !! the weird thing is my car does not used any oil after oil changes !!
what to get some ideas to see what can cause that much hydrocarbures at minimum idle!! before I get crazzy and take engine appart for overhauling

sounds like your running rich.

Check the ICV and check for vacuum leaks. Go through www.clarks-garage.com for how to's

High HC is lean misfire.
Might consider finding a vacuum leak, or spark problem.

Too rich, makes high CO.

sorry but what is ICV ??
I check clarks and went over every vacuum line ! mines are fine!!! I have race cam?? and adjustable fpr ?? but pressure is set to 40psi >>>

icv...idle control valve.

Just looking at the vac. lines wont tell you if they are good or not. try disconnecting them one at a time. when you get to one that doesn't effect the engine then start troubleshooting from there.

Or take the easy route and just replace them with the lindsey racing kit. best $40 you will spend.

a poorly designed chip will do that too

O2 sensor old?

is the cat full size, or one of those silly high-flow units? those do not do as good of a job by a long shot

actually I install two cats one of magnoflow one new at front and other a little bigger new design for 6 cylinders nissans!! car has more torque!!

ok I was reading in clarks about idling stab screw, didn't understand exactly how adjust he mentioned for newer cars, jump some terminals in diagnostic socket, with wire and light?? so when it is light on adjust the screw is that right or I'm lost!!!

You installed 2 cat converters? Why? Hate to sound rude, but if you want a v6 Nissan, go get one. Otherwise remove the extra cat.

sorry that was just to pass inspections ones a year !!

That screw adjusts the idle speed. I would not mess with that unless you idle speed is out of range.

I always thought high hc readings meant that the combustion temps are high. This would mean you are running lean. Vacuum leak, ICV, DME chip and O2 sensor could all cause this. Also, octane and brand of fuel are you using. Don't use an Ethanol blend, it is a hotter burning fuel and could be messing with the DME. Alot of gas stations have switched to 10% ethanol around here. As far as I know Shell is one of the only with straight gas.

What is your idle rpm?

Maybe some one else has tried this and could chime in.... Maybe try a fuel additive like "No Lead". Leaded fuel burns cooler, so I guess an additive that simulates lead would also cool combustion temps, therefore lowering the HC levels.

thanks I want to lear haow to tune this idle screw right ! because i know couple of the mecnanics (not porsche guys) where messing with this , and the manual fule regulator putting gas pressure up down! in couple of the gas testes, my idling is 1200rp , I want to ask also if a bad dme tempeture sensor can give some of this higher hc, becuase this sensor is just $17 at pelicans!! guess I will order new 02 sensor, new dme tempeture sensor and maybe someone here can detail and explain me in easy way how to adjust this screw the right way! I noted also there is other screw by the throttle body has a yellow cap!! what is this screw function?

Hello There:

My 1987 944 n/a was having trouble with very high HC at emissions test.

Turns out the DME SensorII needed to be replaced. This sensor is located in the front of the engine just under the air boot on the engine block. The sensor can be gotten for about $20.00 here from our wonderful sponsor and host.

Hope this helps.