I have an '89 GTA with the 5.7 - stock except for Cat-back

keep hitting 85 on a 55 spec for HC during the driven portion of the test (around 1200 rpm) other numbers are:

Driven CO .03 on a .31 spec, NO 64 on a 612 spec

Idle HC 32 on a 200 spec, .03 on a 1.00 spec.

Runs clean everywhere except on the driven HC

Here is what is done so far:

New NTK o2, fresh oil change (twice in 500 miles), coolant sensor, plugs, wires, Dist cap and rotor, MSD coil, MSD ignition, new intake/plenum gaskets, cleaned the TB, air filter, new 190 t-stat, 5 degrees timing, new style injectors - bosch, new fuel pump, AIR valve, TPS. After changing parts - the numbers rarely vary much. Idle RPM and TPS set to spec.

Car was neglected until I got it recently - runs good but does ping under load (even with timing at 3.5 deg). has 16-17 in vacuum at idle. I can detect a slight surge/miss at light throttle load.

Thinking of adting an adjustable fpr and dropping the pressure before the next test - it should drive up the low numbers - just hoping to lower the HC enough to pass.

Any thoughts?

 

Welcome aboard thirdgen.org.

Since your CO levels are low, you aren't having a rich mixture problem. All the changes you made with no change in readings supports this. So, lowering the fuel pressure won't help, either.

High HC is unburnt fuel. Since you've gone through the ignition system, that indicates you have a mechanical issue such as a leaking exhaust valve. You mentioned a ping, which is often an EGR issue, but since your NOx is low, that isn't consistent. No other noises? Misadjusted valves is one possibility.

A compression test may tell you something.

 

Might look into the cats. I dunno how long it takes the stockers to foul up, but chances are they're still original, and 21 years old. A band-aid solution might be to try gapping your plugs one size up. On a fresh ign system, it should have no problem still making spark. Some people might say that's foolish, but I've heard too many stories where that actually helps.

 

The EGR makes some sense - if it is flowing too much back into the combustion chamber - that would explain the ping and the stumble (lean misfire). The car was scoped and I was told the ECM is dialing in more fuel which was thought to be a vacuum leak on the drivers side (gaskets since replaced with no impact to the numbers). Lean misfire + more fuel = high HC.

The egr was vac tested and it does move but doesn't hold the vacuum for long.

If I can get the car out in the next couple of days I'll disconnect the EGR and plug the line to see if the stumble/ping goes away.

Comments?

 

You still don't understand. Adding fuel is a rich mixture. That leads to high CO.

High HC is unburnt fuel, which is a miss.

 

Bear with me here...

If the EGR is allowing excess exhaust back into the combustion chamber - causes the ping from CC being too hot, slight lean miss.

If there is a slight miss - unburnt fuel.

Plugs showed light tan - computor showed extra fuel added - possible being diluted by the extra exhaust being allowed back into the CC.

When in open loop - no ping or miss.

I'll be doing the compression test later this week - it is a major pita on these cars - but all of the plugs were the same and clean when changed - although they were on the light side.

Yes the HC is from unburnt fuel (or oil) but there is no blue smoke on decel or startup. I'm thinking the ECM thinks lean, fattens up the mixture (per comp readout), something is still leaning the mixture beyond what the ECM can control (leaking EGR or?) leaving the miss and thus the unburned fuel (High HC). The ping is there even with the timing down to 3 degrees.

The car was scoped on the day of the E-test - High HC low CO, extra fuel added per diagnostic. we aren't talking sky high HC here - 85 on a 55 requirement.

I appreciate the help - I agree the compression test will add more peices to the puzzle...

 

It was the EGR - replaced it with one from NAPA (they list 2 but only 1 is correct - the cheaper one - go figure) and the ping and surge/miss went away. Passed Emissions today. I never did the compression test - but the vacuum at idle was 17 inches and steady.