edman

So my 1995 Z28, automatic, has had this lean surge cruising between 1200 thru 2100 rpm at stable speeds, but it clears up if I apply a little throttle loading. Shbox egr problems reads that there could be surging at cruise speeds, ok, I have surging, and my idle can get rougher sometimes when the engine is warmed up. Obviously I am getting a little too much egr at these low speeds, but how do I stop it? This may have been going on for many years, as I always miss or pass smog HC by a couple of points ( due to the missfires at surging ). Would it likely be the solenoid, or the egr valve itself? Has anybody actually had this issue and be willing to share what their problem was and how it was fixed?
My car is quite strong under throttle, but cruising and smogging it has always been a pain, so as I get my hands dirty, some background info would really help! thanks , Edman

Fatdog2

Hi edman.I'm in the L.A. area.Sounds like maybe a vacuum leak.How many miles on the car?The hoses and wires on these older cars are going south as time goes on.The computer regulates how much gas and air is needed and is fed info from the sensors.Do you have an oil leak down the backside of the engine.I did on my old firebird and after I had the intake manifold gasket replaced my surging stopped.Also hoses leading up to the egr could be leaking.I would have the car looked at by a seasoned mechanic.Don't take it to a dealer.They can't do anything unless there is a code present.Might find a corvette mechanic near you.Let us know what it was.My02.

Injuneer

EGR does not control HC. It's there to control oxides of nitrogen (NOx). But if it's causing misfires, could increase HC.

The EGR valve could be sticking slightly open. Pull it off the back of the intake manifold, check the pintle for carbon buildup. Check the hole in the back of the manifold for carbon buildup.... hard to do, but an inspection mirror will help. If it was a major leak, the idle would be extremely rough, to the point of possibly stalling.

The PCM pulses the ground to the EGR vacuum solenoid to pulse width modulate the percent the valve is open. It's conceivable that the solenoid is not closing properly. Try driving the car with the vacuum line removed from intake manifold, to prevent the valve from opening if the solenoid is leaking. Be sure to cap the nipple on the manifold for the vacuum line.

Just to understand the system, the EGR system does not operate at idle, not at WOT, and not above 3,500 RPM. It is used primarily when cruising at low load/steady speed.

edman

Yes, egr does not control HC, unless egr causes surging ( which are missfires), then each miss is a load of HC, and with my smog test RPM in the 1500-2100 range, that's when it surges. I just unplugged my egr solenoid electrical connection earlier this morning and drove a large loop with good idle, good cruise, it was how the car ran fifteen years ago. Then I plugged the egr electrical connection back in, drove the same loop, and have the surging again: slightly surging idle speed, mild bucks and shudders loafing between 1200-2000 RPM between 15-40 MPH. Once past 2100 RPM, everything smooths out. It makes me wonder, could the solenoid, if faulty, send too many pulses to open the egr valve?

Injuneer

Pretty much what I said. Not sure why you want to quibble about it.

shoebox

Perhaps the solenoid is not closing and allowing vacuum to the valve when it is not supposed to. Put a vacuum gauge on it.

edman

So I replaced my egr solenoid, and earned a few cuts along the way. My idle became more stable, but I could still sense mild surging in the 1200-2000 rpm range at loafing along speeds. But, I passed smog today! I have been using a cat cleaner for several fill ups, and today, with a quarter tank, I poured in another bottle, drove two miles in second at about 4000 rpm, everywhere else in a gear too low, got the engine plenty warm, and passed as good as when my cat was new eight years ago! If I had a custom tune, I might have done better, but for what I have ( comp 501, 52mm, f mstr, cai, ign, rlr rkrs ) I feel pretty lucky! HC =31 and 24 (@25 mph) , CO= .01&.01 , and NO= 78 & 68 ( mostly likely my overblown egr ). My best numbers ever! last time, HC was 57 & 32.