EGR affected by exhaust size?

[Indabay]

My 91 240sx ka24de wont pass smog and my mechanic has tried for a month to get it to pass only to tell me he cant fix it and i should replace my 3" exhaust with the origional (OEM) exhaust system. He actually used an older exhaust i had with 2.5 in" piping and cut off the canister tip to replace with a much smaller generic muffler. This had lowered my nox emissions by 50% puting me in range of smog pass by around 60ppm nox. And yes i was well over being a gross polluter before hand.

My question is for anyone who has had a similar problem or is familiar with how the EGR system is effected by the exhaust. Does the size of the exhaust and amount of backpressure really make a signifigant change to the egr system?

[one_eighty_180]

i really dont think it should. but if your numbers changed with a change in your exhuast then thats something to look into... but NOx is from heat ... excessive heat... is that the only thing over?

[Indabay]

he told me that i wasnt getting enough backpressure with the 3in. But
He also bypassed the egr vacuum control modual, he called it a backpressure transducer, and said it actually passed a pretest. but then replaced the modual( sorry my spelling is horrible) thinking it was faulty and said it still wasnt passing.
I have had this car for a year and have had nothing but problems with smog so any help is much appreciated

[Def]

Yes, the EGR works off the static pressure in the exhaust and a 3" exhaust will have almost no static pressure except under very high massflow rates.

Why not run a small diameter silencer and see how it reacts?

[EAT Motorsports]

why do you have 3in exhaust on an NA 4 cylinder if i might ask? basicly back pressure will help the egr take more gases into the tube.

[markyboi]

it shouldn't

i had a 3" exhaust when i had my other 240's smogged...and they all passed with "flying colors"

[S14DB]

why do you have 3in exhaust on an NA 4 cylinder if i might ask? basicly back pressure will help the egr take more gases into the tube.

Cause Even NA the 3" makes more power and TQ then a 2.5". Search.


EGR should work with a header and 3" exhaust with test pipe. Him bypassing the Transducer leads me to believe that your EGR valve is on it's way out. It's ether sticking or the diaphragm is shot on it and it's not opening all the way.

Replace the EGR valve and you should pass easily.

[MrChow]

I have a 4" exhaust and EGR isn't working and I passed with "flying colors too"
Only reason I didn't pass cause of the EGR and gave the engine check light.

[Indabay]

I checked to see what type of pressure was going to the egr from the vaccum lines and it seemed like it would build vaccum and then release like a pulse(couldnt maintain the normal PSI)but there are no holes in any of the vaccum lines. So would you say this is a sign of a malfunctioning egr or a backpressure problem caused by the larger size exhaust?

[S14DB]

Sounds like the EGR is sticking.

[Indabay]

Thanks for your input s14db. I think im gonna try the exhaust since i could use a stock one for daily driving and if that doesnt do it im definetly swaping egr's.

[miklos]

My exhaust is 2 1/4" and I was told today that I need more backpressure to open the EGR.

[S14DB]

My exhaust is 2 1/4" and I was told today that I need more backpressure to open the EGR.

To bad the EGR works off vacuum not "back pressure". The valve is pulled open from a vacuum source. With zero "back pressure" the manifold vacuum should easily pull the valve open and then suck exhaust gasses into the manifold to be re-burned.

Isn't stock 2.25?

[miklos]

The actual words were the Backpressure Transducer needed exhaust back pressure

[240_fun]

NVM msglnth

[S14DB]

The actual words were the Backpressure Transducer needed exhaust back pressure

The EGRC-BPT Valve? If so, that's the exact opposite. It keeps excessive exhaust flow from the EGR valve. To much exhaust pressure from a restricted exhaust system can affect it by over flowing the EGR. Normal or low pressure it doesn't operate.

EGRC-BPT failure is usually a vac leak in the EGR/EGRC-BPT vac lines. Second to a failure or leak in the valves diaphragm.

[shinobis13hb]

i know this is kinda off topic but if the egr is bad and throwing the code coudl it effect idle performence or anything like that?
im having the same problems as him wont pass but i got that taken care off.
but i noticed my car idles kidna ruff or misses sometimes after it warms up. im aware the egr runs off vacume thats why im curious

[miklos]

The EGRC-BPT Valve? If so, that's the exact opposite. It keeps excessive exhaust flow from the EGR valve. To much exhaust pressure from a restricted exhaust system can affect it by over flowing the EGR. Normal or low pressure it doesn't operate.

EGRC-BPT failure is usually a vac leak in the EGR/EGRC-BPT vac lines. Second to a failure or leak in the valves diaphragm.
When the tech told me he needed to take out the injectors and inspect their calibration and flow patterns; I started thinking they don't plan on really getting my car finished. He said 2 injectors were rich because they were the wrong color dots for the month that the engine was built. Screw it...I'm getting a waiver........