High HC on emissions test: vacuum leak, open egr or worn rings?

Old 01-23-14, 02:25 PM
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High HC on emissions test: vacuum leak, open egr or worn rings?

The car: 1994 Mazda 626, automatic, V-6 2.5l 152,000 miles
owed since 2006

The car has high HC on test and even smells like a pre-pollution control car. There is a problem, but it has no drivability issues and no stored codes. My first guess is a vacuum leak, or maybe the egr valve is cracked open just a bit or the rings are going. Unrelated issues include registration is due in April and the car will need tires in 2014. Additionally, the power steering pump is sucking air on cold mornings, one brake caliper is leaking and one of the control arm bolts won't stay tight. Before addressing any other issue I hope to resolve the HC issue or at least pin down the problem.

In Jan., the car failed the two-speed idle test the first time as follows:
156 ppm HC @ 2402 RPM Pass
246 ppm HC @ 710 RPM FAIL (220 ppm is max allowed)

I changed the oil and air filter and cranked up the idle and passed on retest:
124 ppm HC @ 2629 RPM Pass
195 ppm HC @ 1068 RPM Pass

I since set the idle back down to spec.

In November I replaced the plugs, plug wires, cap and rotor. The plugs looked ok. The ceramic part was white to light tan color. I also replaced both O2 sensors. One had a bad heater(open circuit) and I replaced the other one for no good reason. Both cycle from 200 to 800 mV as I believe they should. I checked the fuel pressure and it was to spec. I had suspected a leaking injector but the pressure was retained well above spec when the car was turned off.

In December I got a CEL relating to TPS. It would not stay adjusted, so I replaced it in Jan. Apparently two pins on the TPS single a closed throttle. The circuit is closed with .006 feeler gage an open with a .020 feeler gage as per the book. The intake air boot was also replaced in December.

After the emissions testing I first suspected a vacuum leak. Plastic vacuum tanks and a maze of vacuum lines under the intake manifold are suspect. Pulling it is a big job. I get 13" of steady vacuum at all speeds. Vac. seems lowish, but I do live at 4000'.

I pulled the plugs again and they still looked new and clean. I did a compression test and got the following, going from 1 to 6:
165, 170, 170, 165, 165, 165 w/ each going to 90 on the first stoke and topping out after about 4.

I added 3 squirts of oil to each cylinder and repeated:
212, 205, 202, 202, 205 220

The Mazda book says the range should be 143-203. Dry, I am right in the middle, so OK???????
Variation with oil maybe only indicates the rings are original??????

I can find no spec for vacuum other than 17-22 is normal for most cars at sea level, so 13-19 is OK for 4000 feet above???????????

The car idles fine, so I don't want to suspect a slightly open EGR, but I wonder what the symptoms would be? High HC with low vacuum?????

Maybe a clue? The cat is not original and this car has no downstream O2 sensor.

Bottom line: I can take off the EGR valve and intake manifold and visually inspect everything, but I really want to know the rings are not the issue first. If the rings are the problem, I will probably let the car go.

Lots of speculation and questions above. Thanks in advance for your input.
Old 01-25-14, 03:20 PM
Join Date: Jul 2013
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i think that's a bit harsh

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