My 88 GT just failed smog. The HCs were slightly high at 1500 RPM (99 Measured, 83 Max), but OK at 2500 RPM (59 Measured, 65 Max).
My car has 220,000 miles on it, but I have redone the heads within the last year. At that time, I replaced all leaking vacuum lines and replaced and all ignition components except the coil.
I have noticed a recent drop in gas mileage from about 23 to about 20 MPG average.
Since I understand that HC emissions come from unburned fule, would I expect this problem to get better if I advanced the timing a bit?
Any other suggestions?

If you have not replaced the 02 sensor maybe that would help.

I think my O2 sensor is not very old, although I will check.
Does anyone know about the relationship between timing and hydrocarbons?
I suspect that advancing the timing would help hydrocarbons, butu I have not seen it stated anywhere.

Get your car scanned. Make sure the coolant temp reading on the scanner is reasonable.
If the coolant temp sensor is going bad, it can lie to the PCM about how cold the engine is, making it run excessively rich.

Try seafoaming your engine. I've heard it can help immensely.

O2 sensor and coolant temp sensor are what I'd look at. I'd also put in a new set of plugs (Delco R42TS).
O2 isn't very testable, but if it's old replace it. If it's not, well, under the circumstances I'd probably replace it anyway. Denso sensor from RockAuto is like $15 including shipping. Note: make sure you haven't used dialectric grease at the O2 electrical connector. The sensor actually has ventilation through there - that's where it gets outside air as a reference to compare against the exhaust.
The CTS can be tested by measuring resistance across it's terminals, there's a chart in the service manual (and gets posted on here sometimes) which shows the relationship between resistance and temperature.

CA testing requires the timing to be within +/- 3 degrees of spec, so messing with it is a bit risky, especially since the Fiero timing marks are already rather open to interpretation. I have no idea what the "correct" angle of view is supposed to be - perpendicular to the reference plate? Or the angle you can actually see them from? Who knows. Your timing light might not agree with theirs either.
But I'm not sure if advancing will help HC or not. One thing I've read is that more retarded timing leads to hotter exhaust and a hotter catalyst. So I don't know which direction is best for reducing HC.
California's procedures are pretty much designed to test with the catalyst cold. So if retarding heats it up much more, then that might be the better approach.
Advancing will increase the N2O, so that could be a problem if it's already close.
I had a ridiculous hunting idle problem that made my car untestable, and could only be fixed by severe retard, so I resorted to a chip. If you do much at the distributor they will fail it.

[This message has been edited by armos (edited 10-28-2012).]

Very good suggestions guys, I will try them

I had issues with high HC on a Fiero for several years. Tried the usual stuff (mostly mentioned above) and finally replaced the catalytic convertor, which dropped the HC readings well below the upper limit. If the items listed above don't help enough, you might consider doing the same thing.........

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Rod Schneider, Ball Ground, Ga.
"You can't have too many toys!"
1988 Fiero GT
1987 Porsche 944S
1985 Porsche 944
1966 Porsche 911
2001 Chrysler 300M
Van's RV-6 airplane-at the airport, nearly ready to fly

What I did to pass emissions with my '84 was drive the tank until I had like 2 gallons left, then put 2 gallons of E-85 in the tank, and drove to the emissions station. Dropped the HC from 256 down to 93, and only cost me $5, so I'd have time to fix the real problem.

I hooked up may scanner and the coolant temp sensor seems to follow my aftermarket gauge, So I am ruling that out.
I theorized that a vacuum leak might be my issue. I figured that a vacuum leak would fool the MAP sensor into thinking there was more load on the engine, but its effects would become less as the throttle was opened. Thus I would fail at idle but be better at higher RPMs.
So I started by looking at the output of my MAP sensor at idle. It was 1.23 Volts.
In the process of tryiong to figure out what vacuum reading 1.23 volts meant, I tried the MAP sensor diagnostic in the factory manual.
It says to my MAP sensor should read 3.8 to 5.5Volts with the engine off and 1.2 to 2.3 volts less when I apply 10in of vacuum.
My readings were 4.7 volts with engine off and 2.9 volts with 10 inches of vacuum applied. That means a difference of 1.8 volts which is within specs.
So then I hooked up the vacuum gauge and found a steady 18in at idle (about 1000RPM).
But I can definitely smell the unburnt fuel at idle, and I notice black soot building up on my rear bumper over time.
Any theories?

TPS volts at idle, should be ~.480

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'97 Ram SS/T
'87 Mustang GT- 7730 w/S_AUJP V4
'86 Fiero SE- 7730 swap in progress
'85 S10 Blazer- ARI 3.4; GT3582R

In a separate post, I asked if 18in at idle was what I should expect, and I received a resounding yes.
So can I assume that I do not have a vacuum leak?

I can see from the integrator and the block learn that the computer is adding fuel.
What else can make it do that?

adjust your tps voltage down to .480V at idle, see if that helps

Looks like you don't have a vacuum leak.

A bad reading from the 02 sensor could be triggering the extra fuel.

[This message has been edited by Gall757 (edited 11-10-2012).]

I changed the O2 sensor but I do not see anything in the real time data to suggest that anything is better.
The intergrator has gone up to 144, which I think means it is now adding more fuel.
I tried to adjust the TPS but neither my factory manual or the Chilton manual mentions anything about how to adjust the TPS.
Can the TPS voltage be adjusted on an '88?

the mounting holes should be slightly oblong, the screws can be loosens up and the sensor can be clocked. if you look at the TPS, theres a tab that makes contact with the throttle shaft, the tab can be bent slightly to adjust voltage. since the TPS voltage is high, the ecm isnt using idle spark or fueling, which is probably why the blm is high. once the tps voltage is ~.480V at idle, it should fix the problem

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'97 Ram SS/T
'87 Mustang GT- 7730 w/S_AUJP V4
'86 Fiero SE- 7730 swap in progress
'85 S10 Blazer- ARI 3.4; GT3582R

Next time you go for the test, have only about 1/4 tank of gas, & add 2 liters of methyl hydrate to the tank before you leave for the test. This is an alcohol & provides a bit of extra O2 to the combustion process. This is often enough to allow a marginal vehicle to squeak through.
It's possible too that your cat converter is failing. The OEM units in the first few years of production weren't all that great. They can gradually get poisoned from fuel contaminants & improper additives, too.

I had the car tested again. No luck.
I failed almosty identically to how I failed before, except the my NO emissions (which were not a problem before) were about 1/3 of what they were last time.
My HC emissions came down slightly (from 99 to 94) but did not pass the standard of 83.
So here is what I have done so far:
I replaced the O2 sensor
I changed the spark plugs
I advanced the timing by two degrees

I did not try adjusting the TPS because I could not figure out how to do it, I guess I will try now that I know how.
Until then, I am open to ideas.

http://www.gnttype.org/tech...maintenance/TPS.html


and no amount of special potion is going to fix it as long as the ecm thinks the engine is not at idle

[This message has been edited by 34blazer660 (edited 11-12-2012).]

quote Originally posted by Tekwiz: Next time you go for the test, have only about 1/4 tank of gas, & add 2 liters of methyl hydrate to the tank before you leave for the test..

Where do I get methyl hydrate?
Do I have to worry about being reported to BATF if I do?
Should I not recommend this course of action to my friends with middle eastern accents?

So I just adjusted the tab on the TPS to get my TPS voltage down to .50 volts. Then I took a drive around the block (well, quite a few blocks) and checked the integrator and the BLM. They were both 137 so that does not seem to have made a difference.
Please keep the suggestions coming though, I can use all the halp I can get.

I am having similar symptoms with my car. Do you get a low rpm dead spot getting back on the gas? I also get an intermittent code 45 at idle, and when coasting in neutral with my manual gearbox. In this cold weather my idle will stay up about 1300 rpm and then drop to a steady 900 and triggers the check engine light. I get back on the gas and the check engine light goes back out.

I replaced my O2 and air temp sensors, but not my coolant sensor or TPS. Still have the same issues.

Rick88,
I don't have any of those symptoms. My car runs great, it just fails smog.
If you can get your hands on a scanner, it can tell you very quickly what values are read from your sensors.
I cannot see anything wrong with the readings from mine. But I can see that the computer is adding fuel (because of the integrator and BLM values)

Reset your computer, If you have not.

quote Originally posted by sd_iconoclast: Where do I get methyl hydrate? Do I have to worry about being reported to BATF if I do? Should I not recommend this course of action to my friends with middle eastern accents?

Methyl hydrate can be obtained at any hardware or paint store. It's actually a recommended gas line antifreeze & won't harm your engine in any way. It's also what they run a lot of top fuel dragsters on. Mix it with soap water & you end up with windshield washer fluid.
I wouldn't worry about the boyz from BATF unless you manage to find methanol at a lower price than conventional petroleum based motion lotion.
I wonder how big a group buy that would take?

quote Originally posted by Gall757: Reset your computer, If you have not.

How do I do that?

Does disconnecting the battery do it?

Sorry, can't help you on the chip question...but reset your ECM this way. I think a 1 minute disconnect is enough.

So, based on the Ogre's information, I decided that the PROM with the 3392 ID was the stock one, as it matched the label on the ECM.
I took that PROM out of my formula and put it in my GT. I also reset the computer by disconnecting the battery.
Now my Integrator ranges from about 137 to 144 (up a bit) but the Block learn is down to 128 (from135).
Should I expect my HC emissions to be lower now?

I guess the answer is NO!
I took the car in for another smog test today and it failed again.
Once again, it failed with high HC at 1500 RPM.
This time was slightly better with a reading of 90 PPM. That is down from 94.

So far I have replaced:
Spark plugs
Coil
O2 sensor
PCV valve
Chip (changed back to stock)

Any more suggestions?

Just a thought. What temp thermostat are you running? These engines seem to run a little cleaner with a 195 instead of a 180 degree thermostat. You also might try cleaning the build up off the back side of the throttle body blade. They get pretty gummed up over time. Air filter clean? Just thinking out loud........

[This message has been edited by Rick 88 (edited 11-17-2012).]

One of the best things you can do befor a smog check is to drive it for 20 minutes befor the smog check (longer if its bone chilling cold). The catalitic converter is sort of like a furnace,it heats up real hot,incenerateing any unburned fuel exiting the engine. If you pulled the thirmistat so your engine runs cooler,be sure to put it back for the smog check.

I did drive the car hard for about 20 minutes prior to the test, and the smog guy took me immediately.
But, I am running a 180 degree thermostat.
I guess I will try that next.

any exhaust leaks?

Did you try sea foaming to remove some of the carbon buildup in your engine. Ed from wheeler dealers did a full system flush on a jaguar and brought its HCs from 150ish to like 10. It was crazy and he used some different kind of fuel in the process.

quote Originally posted by 34blazer660: any exhaust leaks?

I could not find any, and I idle at 18 in of HG.

quote Originally posted by sd_iconoclast: ... I am running a 180 degree thermostat.

Replace it (permanently) with the proper 195 F thermostat. That alone may be enough to pass inspection. Remember, all the thermostat does is set the lower end of the engine operating temperature range; it does not affect maximum engine temperature. A low-temperature thermostat is not a cure for overheating.

Have you checked for exhaust maniold leaks ... i.e. leaking manifold gaskets or cracked manifolds? A leak will admit extra air to the exhaust at low throttle settings, which will cause the O2 sensor to indicate a lean exhaust, which the ECM will attempt to correct by commanding more fuel. The high BLMs you report definitely point to this as a possible problem, and I think that's what you're likely to find. Think about it. You know that the exhaust is too rich (visible soot and high measured HC), but the high BLMs tell you that the ECM thinks the exhaust is running too lean.

[This message has been edited by Marvin McInnis (edited 11-19-2012).]

***Final solution***
I finally broke down and took the car to Bruce at La Mesa Auto Care in here in La Mesa, CA.
(8692 La Mesa Blvd, (619)469-1966).

He charged my $150 to diagnose the problem. He determined that my catalytic converter was not functioning.
I replaced the Cat and my HCs aty 1500 RPM went from 90 down to 7.
Also, My CO and NO emissions were both cut to 1/5 of what they were.

Let me give a plug for La Mesa Auto Care.
I cannot recommend these guys highly enough. Not only do they know what they are doing with cars of this era,
but they have no problem discussing the technical aspects of what they found. With most mechanics, I have to pretend that I don't know anything.
Bruce works with me to find the problem.
A mechanic like this is a real find.