Steve Drake

Was smog checking my 1986.5 auto trans to drive to Shark Fest on the 15th. It failed for the fist time in 25 years. The HC (hydrocarbons) are at 108 versus 103 max at 25 MPH part of test. Last time it was at 75. The idle part is 128 max is 128.
So what have I done to affect it.
I changed the value cover gaskets and spark plug galley gaskets. Also the Plugs,
rotors, Plug wires, Caps and Radiator, Windshield, front air dam and tires getting her ready to drive again. Car was down for a few years on Non Op.
Then the car would not run as I did not get the Injection manifold tubes/hoses connected well enough and it was sucking air bad. Reset that and the car runs great. It does idle in gear at about 700 to 750 RPM by ear.
So .....
1. could the MAF adjustment procedure affect the HC at the 25 MPH test. If someone has a spare jumper/led part I would be grateful to buy it. I should probably test this even if it is not my HC issue.
2. Could a small leak in the injection manifold tubes cause a HC increase. In case I still have a very small vac leak.
3. Could the cat converter have crapped out all at once.
4. any other suggestions.

I'll still be coming to the SF meeting on Sat. Wanted to stay at the hotel but have 50 reunion paid for that weekend.

Looking forward to seeing your beautiful cars again and talking about little issues. Will probably drive it there on a one day moving pass anyway. Shes not pretty yet (except to me) but I guess after 25 years shes a keeper to continue fix her up.

Cheers, Steve in Carlsbad

GregBBRD

HC by itself doesn't tell us much, but high HC is a sign of a misfire, which would go hand in hand with an intake leak., plugged injectors, ignition miss, etc.

Post all four numbers at both rpms and it will be easier to give you advice.

Steve Drake

Thanks Greg, Here is the full data set
mph rpm
15 1335 % CO 13.8 %O2 0.6 HC 128 (max 128) CO% 0.57 (max 0.80) NO 763 (1106 max) pass
25 1298 %CO 13.8 %O2 0.5 HC 108 (max 103) CO % 0.39 (max 0.60) NO 608 (938 max) fail

Previous test before work
mph RPM
15 1315 % CO 14.0 % O2 0.7 HC 128 (max 128) CO% 0.57 (max 0.80) NO 533 (1106 max)pass
25 1890 % CO 14.3 % O2 0.4 HC 108 (max 75) CO % 0.39 (max 0.60) NO 282 (936 max) pass

I see the RPM of todays test at 25 MPH was 600 lower than the other test. I wonder if this is a factor.

Cheers, Steve

GregBBRD

Good.

If you look at both sets of numbers, you barely passed the 15mph HC both years. One more ppm and you would have failed.

The 600 rpm difference between the two 25mph tests is bound to have made a difference in readings....

As the cats age, they take more time to "light off". Did you drive the car 15-20 minutes before the smog test to get the cats and the engine hot?

For an O2 sensor controlled car, the CO is pretty high. How old is the O2 sensor?

Have you done a smoke test on the intake system? Keep in mind that there are going to be pieces that are over 30 years old. Pretty important to make sure that there are no intake leaks.

One of the very common things that occur on these '85/'86 cars is that the two vacuum control valves for purging the charcoal canister have failed...allowing air to be drawn from the charcoal canister all the time. (Note that this can "suck" the fuel tank flat!) The first valve either hanging off of the suspension cross bar or over by the junction of the heater "Y" pipe and the front water hose is a 944 part number....and has a "pricing issue" in the system. A mere $770.00, right now. I'm guessing it should be $77.00 and someone missed a decimal point.....

Anyway, both of those valves need to be checked, when the intake system is checked for leaks.

Steve Drake

Thanks much Greg, Your comments are I'm sure of interest to others too. Smog checks are the worst part of owning any car.
Will research options for doing the checks of the intake system and values of the charcoal filters. Frankly I never knew there was such a filter but it makes sense. They must be a very explosive item after years of saturation. Not sure what you meant about sucking the gas tank flat but did you mean like crushing it? Really?
The car was run a long time before the test and was hot so the cat had plenty of time to warm up.
I hope to get it below the max on HC by testing it in a lower gear to increase the RPM. for the 25 MPH test. Seems like 2nd gear is the right one. My car won't hold 2nd gear at 25 MPH right now even with the stick selected so I am making adjustments. It is worth the cost of a test to see if to higher RPM gets it to pass so I am a bit less pressed as I continue to work on better numbers.
Do you feel the MAF idle jumper/LED tuning procedure will help on the 15 MPH part of the test.
I have never changed the O2 sensor. Guess it will be on my shopping list.
Will add to this topic as I work the number s down. The car runs very strong and I can't feel any misfires bog downs, well up to about 120 where I run out of room on the longest local on ramp. I do want it as clean running as I can get it too.
Cheers, Steve

GregBBRD

It is always a good idea to check and monitor the voltage output from the O2 sensor....even before you change it.

The sensor needs to remain connected to the car's computer whIle testing the voltage....you can probe the wire with a sharp probe and then hook up a volt meter to the wire inside.

A healthy O2 sensor on a healthy engine with a healthy injection system will start initially at about .5 volts. Once the O2 sensor warms up, the voltage will fluctuate from .2 to .8 volts....essentially telling the injection to get richer and once that happens, telling the injection to get lean. This fluctuation takes about 5 seconds for a full cycle.

If this isn't occurring, replace the sensor.

If this doesn't happen after the new sensor is installed, depending on what is actually occuring with the voltage, you will have great clues about what to look for.

Report back your results and we can continue.

Imo000

I think the new windshield is the cause!

Steve Drake

Hi Greg and Imo00 First the windshield turned out to be a nice soft landing spot when I slipped from the garage mez. Not bad in cost or work to fix compared to a broken "me". Even got the windshield with tint and the antenna inside. Thank you 928 Int.
Greg I tested the O2 sensor voltage and think it is OK. 4.8-5.1 cold and in a few minutes 3.0-7.2 on my old meter.


I tested the black wire. The two white wires also came from the same loom out of the center hump but I bet on the black wire that goes to what looks like a fuse holder.

Put my old Sears analyser on and the idle in neutral is 685 to 690. That's 590 to 600 in gear but I think the spec is for neutral.

Clearly the RPMs were 600 too low in the 25 MPH test and a bit of throttle pressure will insure the next test is done like the older one at about 1900 RPM in 2nd gear.
Still the high HC in general is a big concern. Will buy some smoke in a can to try that test next.
My car is a late '86 the Trottle body is below.




Note the plug that covers the adjusting screw (per the manuals) was and remains not there. Not sure if it is really important. I have the 928 shop manuals they show the idle and CO adjustments are under the intake tubes and reachable from above. Not mine. Even the 85 to 86 edit in the manual shows this. Cant imagine how to adjust mine with the air filter on but where there is a will ---. Have not touched it yet as the idle is good but the low speed HC is on the edge of failing as you pointed out. I am beginning to think cat converter unless I have a vac leak. The book also shows the leads for CO/idle as the picture has them marked. Your marking was correct based on their voltages.


Will go get a 12 volt LED. then I can do the CO/idle adjustment per the blinking light method.
More after a Smoke test. Seems I saw smoke in a can for something. Could just use a cig smoker blowing through the power brake line per another topic. Or some other source.
Cheers, Steve

skpyle

Following this, as I am sure to have to deal with it sooner or later on my 1986.5...

Thanks for posting this!


Seth K. Pyle

Steve Drake

Hi Seth, Greg is the one to thank. I am posting because I obviously know very little about combating this problem. I am pretty weak with electrical as complicated as the 928. Greg is breaking it down so well I can at least do the tests. I hope by the time I am done We all understand more about the emission system and chemistry. I just don't trust people to work on my car and just start replacing stuff that might be the problem. With my other cars I can at least read the diagnostic error log. Not having a emission analyser is a double blind.
Gregs experience is terrific and his willingness to help others is vital to this board and the 928 community. Back to figuring out how to do the smoke test.
Cheers, Steve

skpyle

Hi Steve!

I very much agree with thanking Greg Brown for sharing his knowlegde and experience. My point is thanking you for posting YOUR experience. Now I, and any other Rennlister who reads this thread, can benefit from this.

This has already been bookmarked for me.


Seth K. Pyle

Steve Drake

I finally got to post #40 Greg referenced. His is a response a few responses down from the topic beginning. It has a very detailed description of the Idle and CO test and a couple really good pics. While I could not find an idle adjustment screw as described in my Porsche manual, Greg has a picture of a driver placed over it. So down the rabbit hole I went.


Still could not see it because a wire and vacuum line completely covered it. Pushed them aside and there it was.


Too bad I knocked the vacuum line loose It took some of those tools you see at the proctologist's office next to the black leather mask and the red rubber ball to get it back in place.
Anyway made the cable tool for adjusting the CO with the air filter in place. used KS brass tubing for the joint. left the cable long and it turns the screw fine as my cable was coated. Also made the 12 volt capable LED for CO test from terminal A and the 5 volt LED for testing the WOT switch from terminal B. I did not have the resistor I needed for the 12 volt so 4 red LEDS in series does the trick for that.
Now one last question for Greg or another that has knowledge of the test.is for the WOT test which ends of the LED go to where. just want to keep from going 2 steps backward if I can. Also if the A terminal reads 12 volts why am I connecting the positive lead to the jumper 12 line and the negative side of the LED to the A terminal. Seems to me I would be doing the opposite positive to A and negative to ground but That is sort of question a beginner asks.
On the smoke testor for looking for vac leaks. It looks like I will build a smoke generator from an old bike pump and a cigarette in the exit tube hooked to the Power brake tube.
I hate buying stuff with all the crap I have laying around. The hard part is finding a smoker.
More to follow.
Cheers, Steve

Steve Drake

Today was two steps forward and 5 back.
Tried to set the CO using the blinking LED and had a few problems. First I did the adjustment with my air filter off. Just got ahead of myself but did got the process to work according to Greg's instructions. Then I tried again with the air filter on and could not bring my idle to 680 even with the Idle adjustment screw at the clockwise stop. Also could not get the LED to blink as I did with the air filter off unless the MAF was tuned next to stalling the engine. I thought I would check my linkage in case it was somehow was holding the throttle body open. In the process I knocked the linkage to the throttle body off. So I had to pull the manifold. Just as well as I broke a couple old vacuum lines. Maybe I can rig replacements and still make the Shark Fest.
So I will start with the Ohm test of the LH cable and be sure I am still close to the basic 340 range. Then check for Vacuum leaks and try to set the idle again. I share some of this to show things don't always move forward. This is one complicated car but sweet to drive.

Steve Drake

Got the intake maifold back together after reattaching the linkage to the throttle body. and replacing a couple old brittle hoses.
Just a quick side track. Below is a trick I have used to replace molded hoses in a situation when you can't get or wait for the molded hose up to about 1".
Just a straight piece of hose and a couple fittings soldered together as needed will get you on the road till you get the correct molded part. Works for all kinds of lines in a pinch.



Hoping it will lower my high Hydrocarbons a bit to acceptable levels I have been trying the set the idle and CO. While I can get the blink light to work when I unplug the jumper between post B and C the idle goes up and won't come down to 680. So I checked the Ohm value according to Greg Browns procedure of measuring this on the LH cable inside the car. My reading is open rather than some reading which indicates the Variable resistor on the LH is working. Should the ignition be one during the ohms test? I did not think it wise with the LH cable unhooked so I did not turn it on. Reluctant to try that without some advice. Headed to Shark Fest this weekend but hoped to keep moving forward.
Cheers, Steve