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Join Date: Apr 1999
Location: vienna,VA,usa
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emissions test failure @ 2500 rpm

i have a problem i'm beginning to try to solve and i admit i haven't gone to all the chat rooms yet to see what info is out there but ....my problem is that a stock 74 2.0L has failed VA emissions inspection at the 2500 rpm test, the test at idle passed, no smoke from tailpipe, etc, all passed. the engine has a stock rebuild (<100 mi on rebuild) of the top end; reused p/c that were honed and checked by machinist; heads tested & passed by machinist). original fi installed and seems in good working order. car starts fine hot or cold. pressure sensor tested for vacuum (sucked (!) on it) and resistance between terminals. throttle position sensor adjusted per instructions downloaded. new vacuum line/hose kit installed at rebuild so don't think there are vacuum leaks, reused head temp sensor (haven't checked resistance, plan to in course), adjusted pot on brain starting at 12 clicks then leaned 3 or 4 until helped idle speed, decel valve installed and seems to work, humm... what else... from what info i've gathered, it seems that adjusting the pressure sensor is supposed to affect the high rpm CO/mixture
although i've read one account to the contrary, (@ Pelican Parts). the plugs are black and sooty and engine performance seems to indicate rich mixture. (hesitation, not great acceleration/smooth rpm increase). so i guess i have several questions 1) i don't know the grind of the cam. engine had big bore kit (96mm)but had d-jet fi. would cam profile affect emissions that much?
2) should i get into the pressure sensor to try that? i switched/exchanged/bolted in the pressure sensor with another one i had and it didn't seem to make any difference. i've read online info on how to do it but hope another 914 owner will tell experience. i've read pressure sensor is 85% responsible for fi running signals. TIA
Old 04-08-1999, 04:37 AM
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The "typical" smog failure for performance cammed or carbed 2.0 motors is a failure at idle. But the failure at 2500 rpms is not unheard of. FYI-The reason Porsche put the FI on the motors to begin with was to be able to pass the 1970s emission regulations without redesigning the motor (big re-tooling $$$). When the air pollution regs got tougher each year, Porsche just added more control devices. Just look at the 75-76 motors with the air pump,the air injectors, the catylitic pervertor, geez what a mess....

Carbs CAN pass the 2500 rpm levels but never at idle. I had a similar problem a number of years ago with a stock 2.0 with FI. If I remember correctly we soldered a radio type resistor in-line on one of the wires going to one of the head temp sensors or something like that. It tricked the brain into leaning the sytem out more at 2500 rpms than the adjuster on the brain box would allow. We had to try a couple of different ohm levels, but we eventually got it.

The trick was given to me from a friend that specializes in air cooled VW/Porsche motors. His name is Billy @ Doug's Bougs in Santa Barbara. You can call him @ 805-962-2503.

Good Luck.
Old 04-08-1999, 08:34 AM
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The first suggestion is to check the ignition system. The points on my 74 got pitted (probably due to contamination) and I failed smog big-time. New points, no problems. Other ignition problems might also affect it similarly.

The black plugs almost certainly indicate a rich condition. I'd suggest checking the resistance of the head temp sensor. I would guess that you'll find it ~1000-2000 ohms when warm. This is too high, you should be seeing ~100 ohms. (Maybe as much as 300.) More resistance gives a richer mixture.

If that is indeed the case, you can replace the sensor. It's not as easy as it looks, and it doesn't look easy at all! I use a deep 13mm socket with a slot cut in one side. The wire goes through the slot. However, the first time I replaced the sensor I stripped the holes in the head. I had to pull the motor, which lead to a full-on rebuild (due to other stuff I didn't realize was wrong untli then).

One mechanic I know tells me he only removes the sensors when the engine is fully warmed up. This is good from one standpoint, in that the aluminum head will expand more than the steel sensor. It's bad from another standpoint, because the heat will weaken the aluminum threads somewhat. It may be worth taking the car to someone with FI aircooled Volksie experience to do this--they should have the "tricks" to help ensure the thing comes out OK.

Another temporary option is to bypass the sensor. You can run a wire from the head temp sensor lead in the wiring harness directly to ground. This will make the mixture fairly lean. The car will not cold-start that way (ask me how I know), so you have to do it after the engine has warmed up. At the very least it can get you past the emissions test and give you time to deal with the problem properly.

Another suspect is your fuel pressure. It should be 2 Bar, or ~29 PSI. Higher pressure will result in a richer mixture.

Good luck!!

--DD
Old 04-08-1999, 09:38 AM
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Dave's right about the ignition. Points and condenser can make a big difference. Also check the setting of the timing as the cam might have wanted more advance than stock which will affect emissions. Finally, I would not do any changes without refering to the smog readings from the test and checking after each change with an exhaust analyzer to see what the effect was.
Old 04-08-1999, 12:07 PM
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Hey! Nice Rack! "Celette"
 
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With a fresh rebuild I can't imagine that it could be as simple as a bad or loose ground could it? This simple suggestion is prompted by a simple experence. Also is your ignition stock? A rebuild sometimes spons the added high output coils. Some of these will require additional resistors. Just a thought.
Old 04-08-1999, 04:46 PM
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I am curious how the smog laws are in VA? In CA you have to have smog repairs done at an approved smog repair shop to have them count towards a high cost waiver, if you finally have to go that route! Repairs you do at home don't count it seems...
Old 04-08-1999, 06:39 PM
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when i rebuilt the top end (that's why i don't know the specs on the cam) i put in new points & condensor. i put in the fi system, all sensors. i did reuse a head temp sensor and will check its specs hopefully this weekend. the pressure sensor i switched was used but vac and terminal resistances checked per specs found this site. i will admit that the timing may be off a couple of degrees at 3500 rpm because it was so &*#%!! hard to see the timing marks (had to remove engine lid to be able to see the marks) but it's close, will readjust to spot on. in VA, i'm not sure that diagnosis counts towards waiver limit, but work has to be done by emissions repair approved facility. i like the fi system but the brain is unfortunately a "black box" thanks for all your help, keep it coming!!!
Old 04-09-1999, 04:17 AM
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well, i checked the resistance of the head temp sensor and when stone cold it measured 3k ohms then drove the car for about 25 min including a run on the interstate ( up to 70ish mph ) then measured the resistance as soon as i got home, measured 0 ohms so it seems to be in specs. on the engine rebuild i put in new points and adjusted dwell to specs, i retimed the ignition as best i could ran the car again sat nite seemed to run only alittle better, hardly noticable, maybe wishing it to run better. so it looks like i'm going to have to try leaning the pressure sensor. also noticed that the cooling flaps control cable operated by the thermostat didn't seem to slack which would close (open) the cooling flaps so have to test the thermostat, etc, etc. don't think that would affect emissions though, just adds to the "to do" list. any suggestions about leaning the pressure sensor? (have seen the site (maybe here) that describes removing the epoxy plug, and CCW to lean, CW to enrichen)
Old 04-12-1999, 04:12 AM
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oh yeah, i adjusted fuel pressure in course of rebuild to 30psi
Old 04-12-1999, 04:15 AM
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if all else fails, go to the autoparts store and look for a product called "all you need" and it comes in a bottle for about $5 you put it in your gas tank and it will gurantee that you pass emmissions or double your money back. I had the same problem (my 75 914 1.8 carbed) did not pass, i put the bottle in and it passed with flying colors. What the hell, if it dosen't work, you can make $5!
Old 04-18-1999, 01:56 AM
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just to share information, as stated in the original post, i am having/had emissions problems in that the emissions was too high. well, i took the car to an emmissions repair facility and one of the things that helped the emissions numbers was a valve adjustment. when i rebuilt the top end, i did the valve adjustment on the bench and in the short time i drove the car, hadn't gotten back to an adjustment (only about 100 miles on engine). well, so i screwed up somewhere, hopefully i'm not that incompetitent that i was the cause of very large valve clearances, maybe some clearance opened up in the running of the engine after the rebuild, but, whatever, i wanted to share this result with other owners for after a rebuild and emissions problems
Old 05-21-1999, 08:05 AM
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Don't give up on your skills just yet. The valve adjustment changes quite a bit in the first few hundred miles after a rebuild, as everything "wears in".

SOP for break-in procedure after an overhaul is to change the oil and adjust the valves as if you were an obsessive-compulsive. 20 minutes @ 2K RPM, change oil & adust valves. 50 miles, change oil and adjust valves. 100 miles, ditto. 500 miles. 1000 miles. 3000 miles. Then every other 3000-mile oil change you adjust the valves. So check them again in a few hundred miles.

--DD
Old 05-21-1999, 09:19 AM
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If you measured the head temp resistence at the sender, and it is ok, try picking up the signal at the brain connection. REMOVE CONNECTOR FROM THE BRAIN!!(else your ohm meter might burn it out) If you have high resistence in the wires, this test will tell you. The 5th (cold) injector could be firing, so unplug it to test. Also make sure all five injectors shut off, remove them from the intake and turn on the fuel pump several times. With out starting. The injector must not dribble fuel (or test with a press. gage, see if the gage hold 30 psi after you turn off the pump.
I have tweeked a press. sensor to work on a 2 liter (it was a 1.7 sensor) But I used a Gunson brand CO meter to check the mixture as I adjusted. Find a smog tester that will let you tweek the sensor while it is hooked up, small adjustments (like 1/8 turn ) make a big difference.
I agree with the valve adjustment, redo it after a break in period, adjust while engine is cold.
Finally replace you points and condenser with a electronic ignition (Pertronix) cost is about $60. and your timing will be dead nits on forever. You will never never have your timing go out again. Highly recommended. The car will also perform better. good luck

dave menche
Old 05-21-1999, 04:37 PM
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