This probably hasn't been asked before, but is there a way to add back pressure to an exhaust system to duplicate the amount of back pressure produced by a stock catalytic converter? Last fall I installed a 1991 305 TBI engine in my 1956 Chevy. It has dual 2" exhaust with Dynomax Super Turbo mufflers. The EGR isn't working and I believe it has to do with the low restriction in the exhaust system. The EGR solenoid and valve test out fine. I'm considering adding some universal cats to replicate more of a stock exhaust but was wondering if there was another way. Emissions testing is not an issue for this car.

 

Why not eliminate the egr?

 

banana in the tailpipe?

Some egr valves did not rely on backpressure for function. I'd check into that maybe, but doubt that's the issue here. What problem exactly are you trying to overcome?

 

I'd like to keep it functional.

 

The biggest benefit to egr, imo, is the reduction of ping at throttle tip in at light cruise (more prevalent with a locked converter). EGR function here can reduce spark retard and improve mileage and driveability. Are you experiencing this issue?

I would not sacrifice a free flowing exhaust if at all possible. How do you know it's not working?

 

Stick a large socket in the tailpipe and test the valve.

 

Experiencing some pinging and slight stumble when accelerating.

So, I just re-tested the EGR system and use the following method for testing, which I found on another forum:

"Shut the engine off and connect the tester directly to the EGR valve. Pull vacuum (around 15 inches is plenty) and be sure it holds for a minimum of 20 seconds. If it does not build or hold vacuum, the valve could be held open by a chunk of carbon, which you could attempt to clean out – but most likely, the valve is bad and needs to be replaced. With vacuum applied, valve held open for well over 20 seconds.

If the valve passes the hold test - with vacuum still showing as holding on the tester, have a helper start the engine while you watch the gauge. The vacuum should bleed off immediately. Vacuum bled off when engine started.

Determine whether the EGR passages in the intake are clogged by carefully depressing the EGR diaphragm with your fingers. Take care to not burn your fingers or get them caught in the openings under the EGR. When you press the diaphragm, the engine should stumble noticeably, and possibly stall. If there is little or no response, the passages in the intake are likely clogged with carbon. Effective cleaning of these passages requires removal of the intake manifold. This is a recently rebuilt engine, so I know the passages are clear. When manually actuated, the engine did stall.

Check the vacuum source to the EGR solenoid. Connect a vacuum tester (Mityvac) to the line leading from the TBI unit to the front (single port) side of the solenoid. There should be full vacuum at idle, around 20 inches/Hg on a stock engine. If not, remove the line from the TBI and connect the tester directly to the TBI to verify vacuum. If none, the port itself may be clogged. Vacuum read 22 in/hg at idle.

Reconnect the line from the TBI to the solenoid and move the tester to the line exiting the rear of the solenoid. At idle there should be NO vacuum reading (be sure you purged your gauge if necessary so that it reads zero before connecting.) Snapping the throttle should show some activity on the gauge as the solenoid briefly commands the EGR open. Did not pass this test. Vacuum gauge needle did not move at all. This is a test I had not done before.

With the meter set on ohms, you can test the EGR solenoid coil across the two pins in the socket; any reading less than 20 ohms indicates a bad solenoid winding. Reading was 43 ohms.

With the meter set on DC voltage, or using a test light, check the EGR solenoid harness terminal A (pink/black) to ground. Key on, meter should read 12V or the test light should light up. Do not pierce the wire to test, simply unplug it and touch the test lead to the connector pin. Meter read 12.36v.

So, from these results, it appears that the EGR valve itself is not the problem. However, the solenoid may or not be working correctly or it may not be receiving a signal from the ECM.

I also found this:

The ECM uses info from the following sensors to regulate the solenoid:

• Coolant Temp sensor
• Throttle position sensor
• P/N switch
• Distributor ( rpm signal )

I need to dig through the wiring harness and see how I have the P/N switch wires hooked up, if at all. Again, this is in a '56 Chevy with a harness from the donor car.

 

I'd pull out your o2 sensor, find a fitting that'll fit and put a pressure gauge on it to see what the system pressure is at.


That should give you an idea of what's going on.

 

Lmao! Beverly Hills Cop status

 

you don't want backpressure.. ever.. it's always bad.. yes, it is possible to go too big on the exhaust and kill low end power, but that's because of overscavenging the exhaust and causign the air/fuel mixutre to blow straight thru the chamber during valve overlap and not because of the lack of backpressure.

you need to tune around the lean misfire- maybe a try a spark plug that's a heat range colder and pull the timing back a couple of degrees.

it might not hurt to decarbon the engine- this is fun.. you do it by trickling water down the carburetor at idle to steam clean the chambers.. you want to pour enough water down the carb that it almost dies, but you want to still be able to keep it running by operating the throttle with your hand as you pour the water.. you will get a whole crapload of steam out the tailpipes- along with chunks of carbon as they get steam cleaned off the piston tops and combustion chambers.. if you really want to have fun (and if you live in the country) you can do the same thing with trans fluid or something like Marvel Mystery Oil.. but it will smoke a lot- and it will smoke for a while- but this will clean not only the chambers and piston tops, but also the back sides of the valves and the intake runners if you shut it down halfway thru and let it sit for a while, and then following it up with the water an hour or so later. you can do this because you don't have a converter to worry about plugging, but if someone with the cat(s) still on the car wants to do it, they might wanna pull the cat(s) off before doing it..

 

You ALWAYS want back pressure, just not too much, no back pressure whatsoever is bad for your engine, the back pressure helps keep air from getting sucked back up the pipe and into the combustion chamber.