Rob Edwards

Got a note from the new owner of my GT today, he took it to get it smogged in the great state of Virginia, and it failed.

Idle: rpm 1013, duration 14.4

HC 112 ppm
Limit: 125

CO% 2.96 (no, not a typo)
Limit: 1.00


2462 RPM, dilution 14.7
HC ppm 111
Limit: 125

CO% 3.994
Limit: 1.00

No visible smoke
PCV system passed
Catalytic converter passed
Fuel evap system passed
Gas cap pressure test passed

First question- Any Virginia 928'ers understand these numbers? Why no NOx number? "Dilution" looks suspiciously like AFR, but that'd be pretty meaningless from a tailpipe sensor. What is dilution?

Here is my CA smog records on the same car.




Anyone ever seen a CO value jump more than 40-fold in less than 18 months? Could this be as simple as cold cats? HC is borderline high, so it must be running rich, but I can't imagine that high a CO value given that HC.

Just to muddy the waters, the car has a temperature-dependent rising idle- when cold it idles at ~800-850, but after 4-5 minutes of running, you can sit at a stoplight and watch the idle bump up to 1000, then 1100-1150 rpm.

New in the last 5000 miles:
Rebuilt LH
Refurbed MAF
New O2 sensor
New ISV, TPS, Hall, CPS, Knock sensors
Every piece of rubber under the intake
New Temp II sensor

I R&R'ed the intake shortly before selling the car in April, in an attempt to cure this slow-rising idle, but it made no difference. So either I incompetently did the intake R&R twice, or there's a funny temp-dependent electrical gremlin in there somewhere.

Thoughts on the sky-high CO value? Hey you Virgins, what do you think?

Mrmerlin

what kind of O2 sensor did you install a multi fit or factory O2?
Have the 2 plastic connectors in the MAF boot been replaced?
Do both of these ,Try another MAF, try another factory O2 sensor.
are the cats factory?

9x8

One of the possible causes for high idle could be throttle not closing all the way (and TPS not detecting "idle" condition) due to overtightened throttle cable (or "tired" throttle spring).

69gaugeman

Based on what you have done I would look at the electrical connections. Specifically the idle switch and ISV. Maybe a crack in the intake manifold? Opens up at temperature? WAG right there, but.....

worf928

Poor ground connection to TEMP-II from water bridge (painted during intake R&R)?
Poor grounds?

A loose LH harness ground (the two at the back of the motor) can cause very bizarre issues.

ptuomov

All theory, no practice below:

High CO means that there are either too many hydrocarbons per oxygen molecule in the combustion chamber or that the combustion is incomplete. CO burns if there's oxygen available, so it shouldn't show up in volume if both there's enough oxygen and if there's complete combustion.

If one of the cylinders is misfiring and the charge is burning incompletely in the exhaust and cat, that might explain it. It could be the spark not firing reliably. Or it could be that one cylinder is too lean to burn because an injector is busted. Perversely, a lean misfire in one cylinder and other cylinders running slightly rich should show up as high CO.

Hydrocarbons don't mean just gasoline. Is it possible that the engine is sucking in a lot of oil from the intake? The oil burns slow and doesn't come from the fuel system.

Air injection working should help with CO, by the way, if the car still has the air pump. Air pump not working wouldn't explain the idle, but it might explain some of the higher CO. If there's a lot of oxygen in there, a working cat should burn the CO.

Now that's theory. In the practical world, what does the Sharktuner status screen say?

Rob Edwards

Factory O2.

New rubber MAF elbow, I probably reused the two MAF boot connectors.

Factory cats, same ones that passed CA smog for 22 years.

Air pump is there, working (AFAIK, the pulley's turning), air injection is connected to the cats.

The only non-stock item on the whole car is the Porkensioner, and the timing was dialed in to 0|0 with my 32V'r.

TPS closes at idle, I get 90 mpg on coastdown.

Water bridge is naked, 99% sure I didn't use any sealant on the new sensor in 2007.

I did all grounds in 2010 when the motor was out, but this is me we're talking about, definitely need to recheck and re-clean all grounds.

Will see if I can get a MAF and another LH to the new owner.

Never had PEM's in this car, so I don't know what the ST status says. Did suggest to the NO (next owner) that a pair of PEMs and a loaner ST with the new idle control algorithm would be an expedient bandaid, but it'd probably be better to find the real root cause.

Thanks for the input!

Lizard928

Has the intake been recoated, or still flaking factory coating?

I'm going to vote for a vacuum leak. Also check brake booster for holding vacuum.

Rob Edwards

Powder coated intake, summer 2007, 7000 miles ago.

Good thought on the brake booster, AFAIK it's the original, though I replaced the check valve during the intake R&R as well.

So perhaps if there's a vacuum leak the ISV can (barely) compensate when the engine is cold, but either the leak worsens or the fueling changes enough with increased temps that it leaks beyond what the ISV can accomodate?

The idle problem has existed for as long as I can remember, I've done two timing belts three intake R&R's, and pulled the motor once. For all the opportunities that I created to FUBAR things up, it's only since the last intake R&R that the smog #'s went kablooie, while the idle symptoms are unchanged.

69gaugeman

If I am not mistaken you can (barely) get to the ISV connector. If you unplug it and the issue remains, the ISV is NOT the problem.

Rob Edwards

I'll add my ISV connector tool to the care package.

blau928

Temp II, possibly the wiring, or sensor itself.. Easy to check the sensor voltage, the ISV, and a few other things with a volt meter, and a jumper lead inside the car at the LH/EZK, and also at the sensor.

I had a fail on a pretest, and it was a ground on the temp II sensor.. CO was out of line, and idle was a bit off. However, not 1000 rpm.. It could be a vac leak, and the LH is trying but not able to compensate for the extra air...

Hope that helps...

Dictys

Just a point of interest mine passed it's emissions test yesterday in UK without CAT's, air pump removed, NBO sensor operational, sharktuned.

Max HC 207 ppm (Limit: 1200 ppm) - PASS
CO% 0.8 (Limit: 3.5%) - PASS

Mrmerlin

the plastic connectors at the boot will crack with age and let in air
how old is the 02 sensor.
temp 2 sensor needs to verified for function

Rob Edwards

Stan-

Thanks for the suggestions, I'll have the NO look at the MAF elbow plastics and send him the link for ohming the Temp II sensor.

One thought he had was a possible FPR or damper diaphragm leak (since the engine has its original dampers that I had sealed up and replated), but he mityvac'ed them and they all hold vacuum.

O2 sensor has less than 5000 miles on it.