View Full Version : "surging"

02-15-2008, 04:28 AM
I came across this tidbit, and thought I'd pass it along, for reference:

Disabled EGR system. (http://meltingpot.fortunecity.com/thurlow/858/rochprob.html)

Neither Karl, Rochester Products Division, or anyone else contributing to this site advocates or in any way recommends altering or disabling your emission control equipment in any way.

That said, those whose EGR systems have "mysteriously" ceased to function have noticed that their vehicle runs differently when EGR is non-functional.

The reason for this is that the EGR pump operates on manifold vacuum.
The primary metering rods are raised/lowered into the jet orifices by a piston that responds
to the manifold vacuum signal. When the EGR is inoperative, the manifold vacuum signal
available to the carburetor is very different from the conditions that the metering rods
and jets were designed for! This typically results in a lean condition, possibly including "surging".

The only real solution is to fix the EGR system.

Those incorrigibles out there who refuse to do this have to have their carburetor re-jetted
and re-rodded to correct for this. Best just to use rods and jets from a pre-EGR carburetor of the same displacement.

Hopefully, Karl will be able to provide the appropriate part #s here in the future.

On a similar note, if the ECM (computer) is disconnected from an appropriate-year
Quadrajet, a severely rich condition results. Again the only real cure is re-rodding.

02-18-2008, 02:35 AM
My 86 Century did this surging when the EGR diaphragm busted. It's fuel injected but the same theory applies to vacuum signals being altered by EGR.

Since I could care less about EGR on this car, I turned off the EGR in the ECM calibration and haven't given it a second thought.

02-18-2008, 02:41 AM
...could care less about EGR on this car...

Consequence...higher combustion temperatures...as they say.

02-18-2008, 02:46 AM
Lol!! If the 13 PSI of turbo boost hasn't raised the combustion temp to dangerous levels; the lack of an EGR is surely not going to hurt it! :)

02-19-2008, 01:34 AM
If the 13 PSI of turbo boost hasn't raised the combustion temp to dangerous levels

But, with 13 PSI, there is an extra fuel burn going on....rick mixture....what articles I have read is suggesting higher temp equal pre-detonation knock....possible...on stock setups.

Btw, when programming, you can't change the number of cylinders?

02-19-2008, 01:58 AM
Oh yes on stock setups the ignition timing is advanced much during EGR operation and you will usually get detonation if the EGR inert gas is not mixing with the fuel air mix.

When you "tell the ECM to not use EGR" it will also not "add advance to the ignition timing" when the engine would otherwise be in EGR-ON mode. So when you turn it OFF in the ECM it is a whole different scenario than simply unplugging the EGR valve.

On some ECMs, there is a setting for number of cylinders. By knowing how many firing events the engine has per 360° of crank rotation, it can calculate ignition advance.

The 1227747 ECM (TBI controller) has setting for 4,6, or 8 cyls in the EPROM (this area can be altered by programming). This is the ECM I have for my draw-through turbo TBI system on the 84 Century Olympia. It came from a V8 truck, so I changed that setting to reflect the 6 cylinder engine in the Century.

The 1226869 ECM (MPFI controller on 86 T-Type) may have the 6 cylinder setting hard-coded in the ECM (can not be changed) since this ECM only came in a Fiero with a 6 cylinder engine. There could be a # of cylinders setting in the EPROM but my programming software definition file does not show this setting.

02-19-2008, 03:34 AM
The 1227747 ECM (TBI controller) has setting for 4,6, or 8 cyls

On Bosch LH-Jetronic II (1984 era) for a 4 banger with 151 cubic inch engine, the spec for a single injector's injected-quanity of fuel is 176 cm3/min (http://www.komfortwaggon.co.uk/injectors.htm).

On TBI, what is the rating of a single injector?