EGR Valve - Why the symptoms?

[sizzling]

Hi All
I recently identified and fixed a problem with my 2010 Mazda 3 MPS (2.3 Petrol turbo) by replacing the EGR valve. I have a reasonable understanding of cars and engines but am not a mechanic. Although I correctly identified the fault with the help of the internet I just cannot understand one of the symptoms. The symptoms where a poor idle and long cranking time when starting.

I understand why a EGR valve stuck open causes a poor idle but I have not been able to find any info to why is causes a long cranking time?

Here is my thinking. When starting the engine the exhaust valves will be closed when the cylinder should be firing so the EGR valve cannot impact compression. Also the engine has not started so there are no exhaust gases so its not impacting the fuel-air mixture? Is this right? What is it that causes the starting problem when the EGR valve is tuck open.

I am asking as I like to understand what is going on but the actual problem is fixed.

Thanks

 

[scuzzycard]

I'm not a mechanic, but I would assume the problem with starting is because a stuck EGR valve behaves like a vacuum leak.

 

[sizzling]

That does seem feasible.

 

[smorizio]

with cars and all the new sensors in them. if one of them fails the cars computer can go into limp mode. default setting that keeps the car for being damaged when driven by a failed sensor that needed. so you can get the car home or to a repair shop.

 

[BadActor]

As scuzzycard suggested. Causes a lean mixture which is hard to ignite.

 

[sizzling]

Yeah I have been thinking about it and I reckon scuzzy is right, it seems to the symptoms. Thanks all

 

[Chicano]

The EGR Valve (Exhaust Gas Recirculating valve) is supposed to open only when the engine has warmed up and I believe when RPMs go up (not during idle). It redirects a small amount of exhaust gas from the exhaust manifold into the intake manifold.

If you only replaced the EGR Valve and did nothing else, it must have had a ruptured diaphragm that even if the valve worked correctly, the leak would add cold air into the fuel/air mixture causing a lean condition when it needed to be rich to ignite under cold engine conditions, and so prolonging the cranking time... and once the enigne had fired up, the lean condition caused the poor idle.

 

[sizzling]

The EGR Valve (Exhaust Gas Recirculating valve) is supposed to open only when the engine has warmed up and I believe when RPMs go up (not during idle). It redirects a small amount of exhaust gas from the exhaust manifold into the intake manifold.

If you only replaced the EGR Valve and did nothing else, it must have had a ruptured diaphragm that even if the valve worked correctly, the leak would add cold air into the fuel/air mixture causing a lean condition when it needed to be rich to ignite under cold engine conditions, and so prolonging the cranking time... and once the enigne had fired up, the lean condition caused the poor idle.

Thanks for the post. The EGR valve on the 3 MPS uses a solenoid instead of a diaphragm, the solenoid opens the valve and a spring closes it. The valve had a build up of carbon and was staying about 4-5mm open and the spring wasn't strong enough to overcome the carbon build up. I changed it for one I got from the breakers off a car with 20k on the clock but having got the old one off it didn't take much to getting working again with a bit of carb cleaner.

I think I follow why the EGR affects the idle but my confusion was why did it cause a long cranking time? When starting from cold there is no hot exhaust gas and is part of what got me thinking what is actually going on. At the moment I think Scuzzycard's suggestion seems to fit the symptoms best for why a long cranking time.
Thanks

 

[Chicano]

The EGR Valve (Exhaust Gas Recirculating valve) is supposed to open only when the engine has warmed up and I believe when RPMs go up (not during idle). It redirects a small amount of exhaust gas from the exhaust manifold into the intake manifold.

If you only replaced the EGR Valve and did nothing else, it must have had a ruptured diaphragm that even if the valve worked correctly, the leak would add cold air into the fuel/air mixture causing a lean condition when it needed to be rich to ignite under cold engine conditions, and so prolonging the cranking time... and once the enigne had fired up, the lean condition caused the poor idle.

Thanks for the post. The EGR valve on the 3 MPS uses a solenoid instead of a diaphragm, the solenoid opens the valve and a spring closes it. The valve had a build up of carbon and was staying about 4-5mm open and the spring wasn't strong enough to overcome the carbon build up. I changed it for one I got from the breakers off a car with 20k on the clock but having got the old one off it didn't take much to getting working again with a bit of carb cleaner.

I think I follow why the EGR affects the idle but my confusion was why did it cause a long cranking time? When starting from cold there is no hot exhaust gas and is part of what got me thinking what is actually going on. At the moment I think Scuzzycard's suggestion seems to fit the symptoms best for why a long cranking time.
Thanks

Right, an open EGR valve or diaphragm leak (depending on the type and damage) would allow exhaust gas (burned or not) or fresh air into the intake, altering the volume of metered air or air-fuel mixture depending of the type of fuel injection.. a vacuum leak would allow fresh unmetered air into the mixture but either (vacuum leak or induction of exhaust gas) issue occurring during idle would modify the appropriate air-fuel mixture for cold start, and prolong the cranking time.