Wierd emissions

[B Frye]

Here's a funny one, not ha-ha funny but odd funny.

Tail pipe emissions for the wifes '76 914 are as follows:

Nearly perfect at idle.
RPM: 988
HC: 44 ppm
CO: %1.10
CO2: %13.4
02: %1.1

However, at high RPM the CO is way too high.
RPM: 2443
HC: 117 ppm
CO: %5.85
CO2: %11.1
02: %0.1

This is directly opposite from what I am use to regarding air cooled engines. I usualy expect a rich condition at idle but good readings at higher rpms.

The engine in question is a 4 cyl 2.0L w/DJet F.I.
All components in the FI have been checked out.
Ignition timing is per spec.
The compression in all cylinders ranges between 140 and 144 psi.
All hoses are good.

The car is stone stock. No deviation from factory specifications.

Any ideas?


UPDATE:

I checked for a vacuum leak and did not find one. Also, rechecked the dwell, timing, and valve settings.

I agree with D.D., I don't want to mess with the MPS. Too expensive if I screw it up.

Maybe the computer control has gone bad.

This is driving me NUTS!

[This message has been edited by B Frye (edited 05-12-2000).]

[John Rogers]

I checked with my smog expert and he said it is a vac leak.

[Joe Bob]

Me three....use some spray carb cleaner, sparingly around suspect areas. A change of idle shows where it is...

[jmcleod]

doesn't the manifold pressure sensor control mixture at other than idle rpm and the pot on the brain control mixture at idle? i kind of went thru the same emissions problem as you have. had to have an emissions certified shop work on car since $ spent go towards exemption, etc, etc. anyway, emissions went down alot after a valve adjustment (i rebuilt the top end and thought i had adjusted the valves per spec) then they adjusted the MPS.

[Joe Bob]

It is my personal opinion that the MPS never needs to be adjusted. To do so, only masks another problem in the system. That is why they seal the adjustment screw to keep us from messing with it....

[Germain]

I just thought I would point out the there is an adjustment screw there. If the MPS never needed adjusting, the screw wouldn't exist... But I agree, adjusting it normally is just a way around some other problem. Hay, if it works...

[Dave at Pelican Parts]

The MPS controls the mixture under all conditions. The idle mixture knob has some influence, but the MPS has more. The idle knob is removed from the equation when the throttle switch is not closed--when the throttle is open, or when the TPS is unplugged.

The MPS can, in some cases, need adjustment. JJ Slowik's old 2.0 had some relatively-minor problems after he got everything else sorted out. His mechanic tweaked his brand-new MPS, and it ran just fine after that. Note that this is a professional mechanic with LOTS of tools (including an exhaust gas analyzer) and LOTS of experience. And this was in Denver, which is 5000+ feet above sea level.

I dunno about you, but I am not in Denver, nor do I have an exhaust gas analyzer, nor do I have that much experience. I'm going to leave my MPS alone.

--DD

[tfast]

Sometimes the charcoal canister can give you problems if it is saturated with fuel vapors. It is suppose to purge only when the throttle is open (ported vacuum), producing high CO on cruise.

[Dave at Pelican Parts]

Uhhh, nope--not on a 914. The cannister has three hoses that go to it. One large hose comes from the fan shroud. Air that has been pressurized by the fan is pushed into the cannister. It goes through the cannister, and returns to the air cleaner through the other large hose. The small hose comes from the "expansion chamber" which sits on top of the fuel tank. This is how the gasoline vapors get into the cannister.

That's all there is to it. No connection to the throttle in any way.

--DD