EGR Valve

57larry · August 20, 2008, 07:59:53 PM

kid has a 86 chevy truck witha 305 and air pump ect disconneted. ran great last couple yrs. now at 35 mph it is starting to backfire. I took a quick look, will change spark plus, cap, wires but the EGR valve is still on the intake but not hooked up. should I take the valve off & put a block off plate on it or just leave it alone. your thoughts please.

wayne petty · August 20, 2008, 08:50:00 PM

a little more info... what kind of stuff is still connected... the feedback carb??? the tbi
where is it backfiring out of exhaust... intake?

have any exhaust leaks... backfiring on decelleration...

check engine light on???

is the coil leaking high voltage out of the sides?

?

coil in the cap.. or beside the dizzy. small cap hei...

so many things... so little info...

check or change the cap , rotor , wires and plugs if you have not done them in a while... examine the sides of the ign coil... look for circular patterns in dust... this indicates high voltage leakage... be sure that if it is a coil in the cap unit.. that you swap over the ground strap that is under the coil... and plugs into the middle of the 3 wire connector.. without out it... bad things will happen to the module.. and coil....

57larry · August 20, 2008, 08:59:10 PM

simple Qjet carb. backfires while acelerating not until 35 mph. no exhaust leaks, regular size HEI. I'll do a tune up. the real question was: what about the EGR valve?

tomslik · August 20, 2008, 09:26:36 PM

Quote from: "57larry"simple Qjet carb. backfires while acelerating not until 35 mph. no exhaust leaks, regular size HEI. I'll do a tune up. the real question was: what about the EGR valve?

what about it?
you said it's not functional....
where does it backfire from?
out the exhaust or thru the intake?
under power or decell?

57larry · August 20, 2008, 09:57:54 PM

does the EGR vavle need to be remove if it's not in use? thanks

enjenjo · August 20, 2008, 10:09:44 PM

Quote from: "57larry"does the EGR vavle need to be remove if it's not in use? thanks

No. If it's not hooked up, it never opens. Unless the guts have rotted out of it.

wayne petty · August 20, 2008, 11:50:23 PM

just diagnosing a back fire is hard enough without wondering if the egr is causing it...... they don't...

tomslik · August 21, 2008, 04:42:23 AM

Quote from: "wayne petty"just diagnosing a back fire is hard enough without wondering if the egr is causing it...... they don't...

well, they COULD.... but not likely....i'd be a lean misfire...
i've got a suspicion the one of the exhaust lobes went flat and it's popping back thru the carb.
just a feeling (without more info)

phat46 · August 21, 2008, 07:39:51 AM

I agree with the flat cam lobe. I had the same truck and had two lobes go flat on the cam. It got progressively worse in a very short time.

wayne petty · August 21, 2008, 11:05:52 AM

what would be almost as bad is when the rocker arm slot wears into the side of the rocker arm stud and falls over slightly, riding on the retainer and bending the pushrod...

when that happens... most people try to stack washers up and pull the stud with a rocker arm nut... this almost never works and usually ends up with a stripped threads .... so when you get there with a proper hardened tool that has full thread engagement . theres nothing left to grab on to...

i am still thinking that it could be a burned spark plug wire.... or ... the cap and rotor are completely wasted... covered with carbon from the coil button...

loose intake manifold bolts could be the cause also... leaking air into the intake ports.. burned out carb mounting gasket... remember there is a bunch of stuff disconnected.. just think what happens if you unhook the vacuum hose to the heat riser valve diaphram... it stays closed... forcing hot exhaust across under the carb... all the time.. not just for 30 seconds or so to warm the carb... this just frys the carb mounting gaskets ...

as for the flat cam.... that is a huge possibility... does anybody remember why they go flat on just a few models... there was a smart fellow who wanted to know why this one motor was eating cams...

he put another cam in it and managed to look down the lifter bore... the cam lobe was not centered under the bore... it was offset... i don't remember the amount... but it was enough that he got buzy and offered special cams through his business that had wide lobes on the affected lobes... seems the bore centerline was mis machined on quite a few blocks...

57larry · August 21, 2008, 03:54:57 PM

thanks everybody for the input. I'll let you know after we do a tune-up

Wild Bill · August 27, 2008, 11:32:07 PM

This is a little counter-intuitive but, here it is.........

Make your EGR operative again. I know this sounds backward but, the EGR makes the engine run cooler. It also actually makes the engine more efficient because it reduces pumping losses. All modern high performance engines utilize EGR. It is something you really want to retain.

Okay, let the blasting begin.......

wayne petty · August 28, 2008, 12:29:20 AM

i'm bored .. so here it goes....

egr flow into the intake manifold reduces the amount of burnable air and fuel in the intake charge... this reduces the temp of the combustion in the chamber ...

if the egr flow can keep the combustion temps below 2600F there will be less or no oxides of nitrogen formed..(NOx)((nasty stuff that burns your eyes))

egr valves should only open above 1200 or 1500 rpms.. below that the engine may not be able to handle it..

they also need to be tested off the car for any leakage through the exhaust side to the intake side.. if they leak... the motor will idle rough..

egr valves in many applications are positive back pressure. this prevents the valve from opening at idle ... the exhaust with the engine above 1000 rpm creates a bit of back pressure as the flow backs up slightly against the converter monolith, when this happens the pressure goes up the hollow stem of the egr into the diaphragm area and closes a bleed port so any vacuum will pull open the valve.. some motors have this in a remote valve to control the egr when there is exhaust back pressure...

A tip... when a car with one of these remote valves fails smog tests (high NOx)... and the egr will reduce the idle speed of the motor... the back pressure valve may be bad.. or the wrong one... or have the vacuum passages in the throttle body clogged limiting the amount of vacuum to the bleed port...

Wild Bill · August 28, 2008, 09:06:24 AM

Exactly right. I assume that you agree with keeping the EGR on engines so equipped? They don't reduce performance but do reduce emmissions and make the engine run better. You agree.