1985 Chevy 305 runs rough with EGR enabled

[petef]

This is a new rebuilt Jasper engine with about 12,000 miles. Engine runs well with EGR valve disabled (vac line disconnected & plugged) , but I'm failing inspection for HC at idle. HC=478 ppm, standard is 200 ppm. Inspected with EGR disabled.

The video below shows two Vac tests, one with EGR DISABLED and one with EGR ENABLED. Engine runs rough as soon as the EGR valve opens which I verified by feeling the EGR diaphram as engine RPM is increased.

If you watch this 2 minute video you will notice the lower vacuum readings with EGR valve enabled. Is this any clue to what is wrong? It did work normally about a year ago judging by a smooth running engine with EGR enabled.

Chevy 305 Engine Runs Rough with EGR Enabled
https://www.youtube.com/watch?v=p1B8...ature=youtu.be

Some other clues that may or may not be related..
* Engine seems to run hotter than it used to. Normal is 180F, but goes to about 200F if idling 10 minutes or more in 80+ weather.
* I may have had a vapor lock condition one or two times in the past year where the engine died as if it lost gas, but it was fine after cooling down.
* I did see some carbon blockage on the EGR port in the manifold when I recently replaced the EGR valve with a new one.
* I'm suspecting that the intake manifold was never properly decarbonized when my mechanic setup the new Jasper engine.

Any ideas or suggestions would be greatly appreciated.
I'm about ready to pull the intake manifold and inspect for carbon blockages.

---pete---

[maxwedge]

The egr controls nox, not hc's, that is unburned fuel,

Misfire, rich carb,loaded up evap cannister are some causes. The egr on most venicles is controlled by
either the ecu thru some tpe of vacuum solenoid. Check on how this should be controlled

[petef]

Originally Posted by maxwedge

The egr controls nox, not hc's, that is unburned fuel,

Misfire, rich carb,loaded up evap cannister are some causes. The egr on most venicles is controlled by
either the ecu thru some tpe of vacuum solenoid. Check on how this should be controlled

OK, the high HC at idle is probably a separate issue from the EGR issue. Let's put that issue aside for now.

My EGR valve is operated by Vac Solenoid which is brand new and operating correctly. The EGR valve seems to be operating normally, because it's closed at idle and opens when the RPMS are increased.

The question is, why does the engine run so rough at about 1500 RPMs when the EGR valve opens? I was hoping the vacuum tests would help with the diagnosis.

Is it normal or abnormal for the manifold vacuum to drop into the RED at 1500 RPMs and above? If it is abnormal, what could be the cause?

---pete---

[maxwedge]

When the egr is open the vacuum will always drop some as the egine stumbles somewhat with no load in neutral. This could be the wrong valve for this engine, some non oe units have restrictors in the box that are used to duplicate the original orifice size.

[petef]

Originally Posted by maxwedge

When the egr is open the vacuum will always drop some as the egine stumbles somewhat with no load in neutral. This could be the wrong valve for this engine, some non oe units have restrictors in the box that are used to duplicate the original orifice size.

I don't believe the problem is the EGR valve itself. Reason being the previously installed EGR valve acted the same way as the new one and at one time it did work properly. By properly, I mean I could rev the engine and at the same time place my finger on the EGR diaphram feeling it open and close in response to the engine RPMs. and the engine would run very smoothly throughout the range.

To me, it seems that something in the engine has changed. The Vac test (decrease in vacuum correlates precisely with the poor engine performance which is why I thought it might offer a clue to what is causing the problem. I guess the only way I'll know for sure is to pull the intake manifold and look for clues such as passages that are restricted due to carbon buildup.

I appreciate your help. If you have any other ideas, I'm all ears.
I'll be sure to post back here on any new developments.
Thanks again!

---pete---

[maxwedge]

Well if the HC's are high the engine is generally not running right and the egr opening just worsens the condition.