nosnow

Looking for ideas.

I have an early 86 that I bought as a project. Car has been off road for years, probably due to emissions failure, it currently runs very strong with no odd behavior such as lumpy idle, hesitation, or willingness to run. I did noticed that I had forgotten to plug the sensor in that fits on the bottom of the air filter assembly. Would that cause the following results?

15mph
HC 84
CO 0.01
NOx 1694 - Fail

25mph
HC 62
CO 0.00
NOx 1622 - Fail

I have replaced the following:
- timing belt & gears w/porken tool gear adjustment
- entire ignition system (coil, wires, plugs (cheap copper ones used), caps & rotors)
- entire fuel system - all rubber hoses, injectors, fuel pump fresh Chevron premium
- intake refresh - all new rubber
- MAF from my S4 which passed smog a few months ago
- new Bosch O2 Sensor

What I have done since:
- double checked intake for leaks - None found
- plugged sensor in

V2Rocket

cat.

nosnow

crap. I was hoping for some cheap fix that I was overlooking.

dr bob

The temp sensor in the air filter housing that you refer to will not normally cause the high NOx reading.

A newer cat might mask the high NOx generated by the engine.

It only takes one or two even partially obstructed fuel injectors to cause the reading you report, even with good cats. Before you invest in a set of cats that will only be a band-aid solution to the symptom, have the fuel injectors professionally cleaned. You report a combination of possible emissions failure, as a cause of having the car off the road for years. Either of those is cause for an immediate injection cleaning effort. There are several good services recommended by members. Injection Labs and Witchhunter spring to mind. IIRC, Roger has already-cleaned injectors with new seals ready to install, available on an exchange basis if you are in a hurry.

NOx is formed when some combustion condition causes a flame temp greater than about 2200ºF in a chamber. This may just be a small section of the combustion front, where a lean condition makes it an explosion rather than a flame propagation within the chamber. The oxygen sensor reads the average CO in the exhaust stream from all the cylinders, and adjusts all the cylinders together to achieve target mixture. One or a few weak injectors receive only a small amount of fuel correction, while all the others get enough to balance the weak ones when measured at a common downstream point.

-----

My '89 was suffering from progressively higher NOx readings as the years went by following purchase at 22k. It ran fine, idled fine, etc. New injectors went in as part of an intake refresh/refinish effort, and suddenly the NOx was back to initial purchase levels again. The idle quality, which had seemed fine before, was excellent again. No vacuum or oil system hoses were damaged, so no vacuum leaks were repaired during the refresh effort although everything went back in new. Overall performance and fuel economy were improved slightly too. I drive the car like a white-hared senior citizen, more rumbling-around touring than screaming blasts these days. Since the car sees winter hibernation now since leaving L.A., it gets regular doses of Sta-Bil with fill-ups. No telling which tank will be in there when the driving season is done. I'm hoping that the Sta-Bil will help with possible injector and other fuel system varnishing and deposits. So far so good after a few winter hibernation periods, and really not very much driving. I put gas in it twice last year, not really enough to say it's close to being the regular driver it was in SoCal.

PorKen

Unplugging the air temp sensor is functionally the same as connecting the 'octane-loop' plugs at the EZF, retarding the ignition at higher load. Shouldn't affect a smog test at low load?

nosnow

I replaced the injectors with new/rebuilt ones from one of our vendors. I'll pull them and send out for cleaning/testing, maybe one or more are acting up.

dr bob

If they are recently new or cleaned/rebuilt you can probably discount what I shared above. On to air leaks. weak MAF, etc. Replace the oxy sensor too. The clean tailpipe readings for CO and HC are good but don't tell you a lot about what's happening in front of the catalysts. Another thing -- using the correct copper plugs seemed to help on mine. As tempting as it might be to spend more on platinum or other more expensive plugs, the copper WR7DC or current equivalents have the smallest risk of hot-spotting. The colors on your old plugs may offer a hint towards which cylinder(s) are not firing correctly.

GregBBRD

Not necessarily a cat....although a new factory $4000 cat works so well that it can mask almost any problem. Truly "bad cats" are super, super rare on 928's. (Out of the hundreds of 928's I work on, each year, I doubt I replace more than one cat.....if that.)

The car is too lean (0.01 CO and 0.00 CO), which drives the NOX way up. (High NOX can be a sign of bad cat or high combustion chamber temperatures from being too lean. (If you have zero CO, it's too lean.)

Bad O2 sensor, bad MAF, or air leak are the first places to look, which it appears you have done. There's a super common air leak past the charcoal canister purge vacuum system that almost everyone misses. Power brake booster holds vacuum? Vacuum pods inside the car hold vacuum? (Almost none do, these days, unless they have been changed.)

nosnow

I took apart the runners on the intake yesterday to triple check hoses and vacuum lines. Put everything back together and started the car, this time when the engine is at temp I hear a slight whistle so I started manually manipulating the throttle under the hood and I apparently moved it a little differently and when I rev the engine I can hear a leak. It looks like the vacuum leak is at the pivot points for the throttle. At idle it is fine but revving to 3K or holding at 3500 for more than 10-20 seconds it starts to whistle and if I abruptly let off the throttle the revs stay high for a few seconds accompanied by a whistle, the car then settles back into a nice idle. This only happens when the car is at operating temperature so either the richer mixture masks the issue or the leak becomes greater with expansion of the metal parts.

I hope I can get the seals for the pivot points.

dr bob

Paul --

Look for a hose off in the intake. Manifold pressure generally increases (vacuum drops) under higher load so it won't be one of your typical manifold vacuum ports. On some earlier cars, the vacuum for the distributor uses a ported vacuum connection in combination with manifold vacuum to offer advance and retard functions. The ported vacuum is stronger as the air velocity through the throttle body increases. I'm pretty sure your EZF car doesn't use that, but the ports may still exist somewhere underheath.

Use a length of vacuum tubing stuck in your good ear as a sound sniffer. Obviously you won't connect the outer end to a vacuum port, but instead use it to do some remote/localized listening to pinpoint the source of the whistle.

IIRC the throttle shaft rides in needle bearings with seals part of the bearing. Replacing those bits involves removing the throttle plate (two screws) that may include Loctite or the screw ends are staked to discourage them from rattling down into the engine. Take pictures as you disassemble so you can get the linkage and return springs back where they belong. The bearings themselves are easy to change once you get to them. My S4 has access to the clamp screws from the passenger side with a long 1/4"-drive extension. That may or may not be the same with the '86 organ-pipes manifold.

nosnow

This is probably the port that had a cap on it in the front. If memory serves me there are 2 ports, 1 capped and 1 that goes to the switch that resides on the crossbar. There is one on the bottom which goes to the slitter for the fuel dampers & charcoal canister and the other to the EGR valve. I have checked for vacuum and leaks at all these point. If I can pull the cap and put a hose on without removing it I will test otherwise I have to pull the whole thing out which is almost as painful as an S4 but not quite.

This car is really trying hard to stay a parts car... the old adage "the most expensive Porsche, is the cheapest Porsche" has really held true!

dr bob

If there's a cap on a port already no worries. Looking for the one that's singing to you, whistling to get your attention. The one that's drawing in the unmetered air.

There are smoke generators that can be used to find intake leaks. Also, Dwayne's intake refresh guide has instructions for making a tester from HD plumbing bits and some air from your small compressor.

Speedtoys

Who needs a smoke machine.


We all have that one friend...that vapes...

nosnow

monkey wrench

Wow that was an interesting read with a happy ending !

Here I’ve Vancouver we had emissions tests and we required it for insurance so it would get worrisome and costly
What we Learned in time was they would only do the idle test not the dyno test if you had studded snow tires so I’d plan to do it in winter months. They didn’t even seem to care if it was just one back tire
they did the idle test but if it was a bit high on idle speed that wasn’t much of an issue and I believed it would misfire less if an old car wasn’t slowed down too much.
the most helpful was a hole in the exhaust they would look at the exhaust with a mirror in a pole but not very closely and a convenient rust hole would make them pass by letting air in or exhaust out before the tailpipe. Thankfully you didn’t need to resort to any of that nonsense !