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My '88 was running badly at low revs in OD, so on advice, I plugged off the EGR, reconnected it for my last smog test and scraped through with MAX HO readings of 112 and 86. It's due again in 3 weeks and, I'm sure, it will fail. All other readings were low. What should I do ? Any help is greatly appreciated.....Roger.
You cite high "HO" readings the last time you passed smog as what has you thinking you'll flunk this time.
What do you mean by "HO".
Smog check tests:
hydrocarbons (HC)
carbon monoxide (CO)
carbon dioxide (CO2)
oxides of nitrogen (NOx)
There is not exhaust emissions test for "HO".
What did your car test at and what are the maximums listed on your smog check results from last time.
If you suspect a problem with EGR, you'll never pass.
The OP states "I rapidly get out of my depth with anything newer than a carbed car." In that case, I suggest you find a service shop familiar with L98 Corvettes and have them inspect and test your exhaust emissions controls and repair as necessary before you go test again.
If you insist on DIY, because you suggest a problem with EGR, I recommend you get the 88 Factory Service Manual and troubleshoot the EGR system using the info in the manual.
Good luck.
The EGR itself is pretty easy....
find the end of the vac line that runs from the egr valve to the egr solenoid on the T-stat housing... engine off.
Get a small dental mirror (or mechanics that tilts/telescopes) and hold it so you can see under the egr valve bonnet. The valve should move freely with the least vacuum....you can suck on the hose enough to make the valve move. It don;t take much. If it moves, great. It should have a "bottom" so that you cannot keep ;pulling air.
If it is hard to move or stuck, it may need to be replaced. That bonnet is a big diaphram that multiplies the vac many times to increase its pulling power much like the brake booster...it'll move a lot and do a lot of work with little vacuum.
Now the solenoid...
99% of the time the vac source is bad...leaking or broken. This is "ported" vac thats only available at certain throttle positions so it must be plugged into the underside of the throttle body. The sticker on the radiator shroud shows the vac route.
IF you have to replace the EGR valve, be certain that you get the correct one. There are similar valves that operate differently. Positive back pressure and neg pressure. I think they used both over the yrs, so look at your specific yr model for the correct egr.
It might not hurt to get some throttle body or carb cleaner and spray it into the PCV hose with the engine running fast idle and into other places to clean out as much gunk as possible. Since the pcv and egr can cause a wet carbon build up inside the plenum and intake, that can add the the HC of the test result. The cleaner the intake the better/cleaner the burn. Use caution spraying so the MAF (if equipped) is not damaged. Personally, I would'nt spray the MAF to save my life. Too fragile, too expensive and too easy to work around it...The burn-off circuit takes care of the MAF housekeeping.
Thanks Hib, Sorry I meant HC. readings were...At 15 mph/25 mph..112/74 . allowed 112/86..... CO. 0.02/0.01. allowed 0.72/0.60....NO.98/75 allowed 778/717. I suspect the EGR as, plugging it off cured the miss I was getting at low revs under load.......Roger.
Your post is difficult to read. Let me confirm...
HC tested at 112/74, maximum allowed 112/86
CO tested at .02/.01, maximum allowed .72/.60
NOx tested at 98/75, maximum allowed 778/717
Is this correct?
The problem is high HC not high NOx. Your NOx numbers are way below the limit so if you don't find a problem with the valve leaking causing misfire, I'd rule out the EGR system.
The high HC could be misfire...it could also be leaking injectors or other problem causing the engine to run rich at idle and light throttle.
In what condition is the engine's fuel system?
Have you run the fuel pressure tests listed in the factory service manual?
In what condition is the engine's ignition system?
How many miles on the engine?
The EGR itself is pretty easy....
find the end of the vac line that runs from the egr valve to the egr solenoid on the T-stat housing... engine off.
Get a small dental mirror (or mechanics that tilts/telescopes) and hold it so you can see under the egr valve bonnet. The valve should move freely with the least vacuum....you can suck on the hose enough to make the valve move. It don;t take much. If it moves, great. It should have a "bottom" so that you cannot keep ;pulling air.
If it is hard to move or stuck, it may need to be replaced. That bonnet is a big diaphram that multiplies the vac many times to increase its pulling power much like the brake booster...it'll move a lot and do a lot of work with little vacuum.
Now the solenoid...
99% of the time the vac source is bad...leaking or broken. This is "ported" vac thats only available at certain throttle positions so it must be plugged into the underside of the throttle body. The sticker on the radiator shroud shows the vac route.
IF you have to replace the EGR valve, be certain that you get the correct one. There are similar valves that operate differently. Positive back pressure and neg pressure. I think they used both over the yrs, so look at your specific yr model for the correct egr.
It might not hurt to get some throttle body or carb cleaner and spray it into the PCV hose with the engine running fast idle and into other places to clean out as much gunk as possible. Since the pcv and egr can cause a wet carbon build up inside the plenum and intake, that can add the the HC of the test result. The cleaner the intake the better/cleaner the burn. Use caution spraying so the MAF (if equipped) is not damaged. Personally, I would'nt spray the MAF to save my life. Too fragile, too expensive and too easy to work around it...The burn-off circuit takes care of the MAF housekeeping.
Dealers:
