1974 EGR Question - L-48 vs L-82?

[Paul L.]

(First let me say that I am glad to be back after an absence of about 18 months. I sold my 1967 coupe in August of 2008 after buying a 1974 convertible. I basically just lost interest in Corvettes in 2008 but things are perking up!)

My 1974 roadster is pretty much stock with an L-48 and TH400. Lovely car with medium red metallic exterior and oxblood interior. A very nice, near-new black Haartz top.

I am putting about in the basement this winter with a #340261 intake manifold that I picked up on EBay. It has been de-carboned and most parts (choke, spouts, etc.) are newly installed. My carb (on the car) is the correct #7044206 and the current EGR is #17051824 (I believe L-82). This is apparently incorrect and the proper unit is #7041409. The latter NOS unit I have found in New York State. It is on its way by US Mail.

My question is very basic: what is the difference between these period EGRs and can they affect performance? I had an off-idle stumble last summer with the ...1824. I realize that there can be a host of reasons for that but could the EGR be one? My TCS has been removed and I am using intake vacuum for the distributor advance and TCS carb ported (t-stat housing thermal) for the EGR.

The car is obviously not correct but still is a fun drive on a sunny Sunday afternoon.

[Edward J.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul, those EGR valves use to give me more grief when I was at the chevrolet dealer as a line mechanic, and now with the gas formulas changing I suppose it could even be worse,they work on ported vaccum, and a themo vaccum switch mounted on the intake, make sure the valve is not operating during cold start-up, a easy check is while driving the car cold and seems to run okay, when it warms up and the hestitaion is there, its likely the problem, , Ive had little luck with swapping the valve out with a new one, no two cars are alike, i've found that one car will give you drivabilty problems with the valve and others don't.

Try the routine checks of the engine spec.s once you remove the intake and take a close look at the intake valves for carbon, to much carbon on the valves act as a sponge to the gasoline, causing the engine to run even leaner. valve seals are always a problem with small block chevys. which adds the the carbon build-up.If all looks good three is one other thing I use to help alittle, that was a vaccum delay valve in-line to the valve, not ncrs but helps.

Paul, I don't know about the correct part no.s , Joe L. can tell you, you may have the replacement part #
it now.

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Thanks Edward,

The TVS (NAPA black) for the EGR is on the t-stat housing and opens I believe at ~120*F. It is connected to ported vacuum on the carb. So no EGR at warmup. And that is correct.

The car is cold-blooded with a divorced choke and I expect that. We are spoiled these days with computer controls. But after warmup the car runs very well with just an annoying "fart" coming away from a stop sign or red light. One of those little things that can drive you crazy and spend $2,000 on an EZ Fast injection system!!

Ignition is points-based of course and set at 36* at 3000 with Sears setback light translating to ~8* static at idle. So that is fine. Idle is 750 Park and 550 in Drive. No stumbling.

I have a delay valve (pic). So you are suggesting putting it in the EGR TVS/carb hose?

[Edward J.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul the valve works the opposite, cold start no operation and warm operation its working, and that vaccum delay is what you will want to try in the hose to EGR.
dwell should be around 30 degrees. timing sound good, once last check, the acc. pump on carb ,should see a good shot of gas when looking into the throat of carb.
Another easy check is to just drive the car with the EGR un-hooked and see how it drives,that will just eliminate the EGR.

[Joe L.]

Re: 1974 EGR Question - L-48 vs L-82?

(First let me say that I am glad to be back after an absence of about 18 months. I sold my 1967 coupe in August of 2008 after buying a 1974 convertible. I basically just lost interest in Corvettes in 2008 but things are perking up!)

My 1974 roadster is pretty much stock with an L-48 and TH400. Lovely car with medium red metallic exterior and oxblood interior. A very nice, near-new black Haartz top.

I am putting about in the basement this winter with a #340261 intake manifold that I picked up on EBay. It has been de-carboned and most parts (choke, spouts, etc.) are newly installed. My carb (on the car) is the correct #7044206 and the current EGR is #17051824 (I believe L-82). This is apparently incorrect and the proper unit is #7041409. The latter NOS unit I have found in New York State. It is on its way by US Mail.

My question is very basic: what is the difference between these period EGRs and can they affect performance? I had an off-idle stumble last summer with the ...1824. I realize that there can be a host of reasons for that but could the EGR be one? My TCS has been removed and I am using intake vacuum for the distributor advance and TCS carb ported (t-stat housing thermal) for the EGR.

The car is obviously not correct but still is a fun drive on a sunny Sunday afternoon.

Paul------


GM #7041409 is the correct, original EGR valve used for your application. GM #17051824 is a GM replacement valve for 1974 with L-48, THM-400 and California emissions.

The 2 most common problems for EGR valves are ruptured diaphragm (which causes the valve not to work, at all, but usually causes no driveability problems but may cause it to fail a smog inspection, if required) and a carbon-stuck valve which is off the seat (i.e. "stuck open"). The latter causes significant driveability problems since EGR is present under all operating conditions.

A new valve will solve either of the above problems.

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul the valve works the opposite, cold start no operation and warm operation its working, and that vaccum delay is what you will want to try in the hose to EGR.
dwell should be around 30 degrees. timing sound good, once last check, the acc. pump on carb ,should see a good shot of gas when looking into the throat of carb.
Another easy check is to just drive the car with the EGR un-hooked and see how it drives,that will just eliminate the EGR.

Edward,

I guess I miscued on my description. Yes, no EGR on cold start but the TVS kicks in to ported at about 120*F.

I have a Viton accelerator pump that I will try as well.

And of course driving without the EGR does make sense. Just to try things out. But the Smog Police here in Ontario will do a $365+ job on you. It was not pretty last summer. I can attach a newspaper article where I was involved....if anyone is interested.

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul------


GM #7041409 is the correct, original EGR valve used for your application. GM #17051824 is a GM replacement valve for 1974 with L-48, THM-400 and California emissions.

The 2 most common problems for EGR valves are ruptured diaphragm (which causes the valve not to work, at all, but usually causes no driveability problems but may cause it to fail a smog inspection, if required) and a carbon-stuck valve which is off the seat (i.e. "stuck open"). The latter causes significant driveability problems since EGR is present under all operating conditions.



A new valve will solve either of the above problems.

Thanks Joe. It's been a long time from my 1979 days. Nice to hear from you. That one is still on the road (pic). The ...409 is new so I do not anticipate any problems (with the EGR) come spring. There may be other issues but I will get back here with them.

[Edward J.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul thats a bummer,and thats a hefty fine. You maybe able to do the test in the driveway with the valve hooked, just put the car into gear and acc. just alittle to see if it hesitates,then with the valve hooked to see if its gone.

Here is Mass. we don't have any smog police, thank god, I don't think I would enjoy paying a fine like that.I work to hard for my money as everyone does.

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul thats a bummer,and thats a hefty fine. You maybe able to do the test in the driveway with the valve hooked, just put the car into gear and acc. just alittle to see if it hesitates,then with the valve hooked to see if its gone.

Here is Mass. we don't have any smog police, thank god, I don't think I would enjoy paying a fine like that.I work to hard for my money as everyone does.

I was in this group, but not inspected. A 1978 with Hookers was. Obviously the cat cons were gone. The Smog Police are not sniffing (these old cars are exempt here from emissions) but looking for any deviation from original factory equipment emissions configurations, strictly a mechanical look under the Canadian federal EPA.



I subsequented contacted GM Canada who verified that a 1974 with L-48 and automatic transmission (non-California) did NOT have an AIR pump. You have a four-speed, yes, but an automatic, no. I have that certification in my car. That was not true in 1973 or 1975 but it was in 1974. I do not know why. Those were strange years. But I carry around that GM certificate to avoid a $365.00 fine. Or if need be to take the 25-year-old emissions zealots to court.

[Joe L.]

Re: 1974 EGR Question - L-48 vs L-82?

Edward,

I guess I miscued on my description. Yes, no EGR on cold start but the TVS kicks in to ported at about 120*F.

I have a Viton accelerator pump that I will try as well.

And of course driving without the EGR does make sense. Just to try things out. But the Smog Police here in Ontario will do a $365+ job on you. It was not pretty last summer. I can attach a newspaper article where I was involved....if anyone is interested.

Paul------


It depends whether they do an actual functional test or just a visual inspection. If it's a visual inspection only, a "bb" inserted in the vacuum line will render the EGR inoperative but, from appearances, the system will appear just like it should.

[Edward J.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul, thanks for the article, by the way nice pic.s of your corvettes, great looking cars.

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul, thanks for the article, by the way nice pic.s of your corvettes, great looking cars.

Yes, I hesitated to come back to NCRS because my Corvettes are OK but not "correct" but we after all are enthusiasts in some way shape or form. My 1974 has a modern stereo with four speakers and a sub-woofer in back. The interior look is Willcox leather-like.

Nice people here. The CF has gotten rather strange. I am a 10-year member but I do not feel very comfortable over there.

[Michael W.]

Re: 1974 EGR Question - L-48 vs L-82?

Yes, I hesitated to come back to NCRS because my Corvettes are OK but not "correct" but we after all are enthusiasts in some way shape or form. My 1974 has a modern stereo with four speakers and a sub-woofer in back. The interior look is Willcox leather-like.

Nice people here. The CF has gotten rather strange. I am a 10-year member but I do not feel very comfortable over there.

Paul,

I'm glad to see you back. Your car looks great, as always. Not sure what you mean about the CF C3 section being 'strange'

[Christopher K.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul, welcome back.

I'm sure I learned more than a few things from your advice on the CF.
My '73 intake and valve are probably identical in function to yours, and so far are working properly.
If you end up searching for an nos replacement egr valve I do see them on ebay from time to time.

Your '74 looks great, keep us posted on your progress.

Chris

[Paul L.]

Re: 1974 EGR Question - L-48 vs L-82?

Paul, welcome back.

I'm sure I learned more than a few things from your advice on the CF.
My '73 intake and valve are probably identical in function to yours, and so far are working properly.
If you end up searching for an nos replacement egr valve I do see them on ebay from time to time.

Your '74 looks great, keep us posted on your progress.

Chris

Chris,

Thanks for the kind comments. I did locate a #7041409 NOS EGR and it is in transit.

Those 1973-74 EGRs are rather odd in that they are not mounted on studs and fastened with nuts as are the 1975 and up triangular versions. They simply sit on the intake and are held in place with a large clamp. An unusual design that I suspect was discontinued in short order for technical reasons.

The pic shows my basement #340261 intake. Wrong colour of course but then again my NOM engine is black. No Chevy Orange at all.

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Another option (non-NCRS) is an electric choke. I do like to fool about.

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