OSM Garage Posted October 13, 2014 Share Posted October 13, 2014 Hey guys Pick at your collective brain if I may. I have noticed with my car (Sigma Wagon) that there is a flat spot between 3,000 and (roughly) 4,200rpm. It feels like as soon as it gets to 3,000 the rate of acceleration dies down and once it gets to (roughly) 4,200rpm its like vacuum secondaries open up and we're off again. The setup is almost entirely stock. Only aftermarket bits used are exhaust manifold, fuel pump (Bosch 910) and the factory fpr has been moved to a different location. Do we think that the issue im experiencing is fuel related? Cheers Quote Link to comment Share on other sites More sharing options...
DriveBy Posted October 14, 2014 Share Posted October 14, 2014 Vacuum advance unit issue? The diaphragms in the unit do fail/split etc.. and then can't advance the timing as the revs increase. Check that and the vacuum hose running to it. Quote Link to comment Share on other sites More sharing options...
Spencer Posted October 14, 2014 Share Posted October 14, 2014 That's not how vacuum advance really works, if you are at WOT there should be no vacuum in the manifold hence no vacuum advance. Its made to apply extra advance under cruise for a better burn/economy. 1 Quote Link to comment Share on other sites More sharing options...
HighLUX Posted October 14, 2014 Share Posted October 14, 2014 Airflow meter? Quote Link to comment Share on other sites More sharing options...
ajg193 Posted October 14, 2014 Share Posted October 14, 2014 That's not how vacuum advance really works, if you are at WOT there should be no vacuum in the manifold hence no vacuum advance. Its made to apply extra advance under cruise for a better burn/economy. Depends where the vacuum is drawn from. On a 4K the vacuum is drawn from above the throttle valve so there is only vacuum advance when the throttle is open. Quote Link to comment Share on other sites More sharing options...
Spencer Posted October 14, 2014 Share Posted October 14, 2014 That doesn't have any relevance to what I said, the throttle plate is still open at cruise speeds and vacuum advance is applied. Ports for vacuum advance that get blocked by the throttle plate at idle are just to stop the extra timing being added in at idle, some factory systems do this some do not. Like I said there is low/no manifold vacuum at WOT so no advance is added by the vacuum canister under this condition. 1 Quote Link to comment Share on other sites More sharing options...
OSM Garage Posted October 14, 2014 Author Share Posted October 14, 2014 Sorry, I didn't explain the motor. Its a 4G63 DOHC injected. I assume the computer looks after timing? Quote Link to comment Share on other sites More sharing options...
DriveBy Posted October 14, 2014 Share Posted October 14, 2014 That's not how vacuum advance really works, if you are at WOT there should be no vacuum in the manifold hence no vacuum advance. Its made to apply extra advance under cruise for a better burn/economy. My bad. Had similar problem in starion which was vac advance. Also assumed entirely stock meant it was original engine. Might just go and peruse the pics and vids instead of trying to hang out in tech. Quote Link to comment Share on other sites More sharing options...
Spencer Posted October 14, 2014 Share Posted October 14, 2014 Wasn't trying to be a dick about it man! Hang around. Vacuum advance gets blamed for lots of things as people don't know what it's doing. Worse thing they do when they fail is create a vacuum leak pretty much. Anyway back to this Mitsi things problem. Quote Link to comment Share on other sites More sharing options...
ajg193 Posted October 14, 2014 Share Posted October 14, 2014 [At the risk of being laughed out of here] Have you hooked it into a diagnostics machine? That could possibly tell you something of importance.Also, if the Mitsubishi manuals are anything like the Nissan manuals it will have a big list of detectable and nondetectable faults which you could go through and check. Quote Link to comment Share on other sites More sharing options...
OSM Garage Posted October 15, 2014 Author Share Posted October 15, 2014 No, I haven't plugged it up to check. Ive put a gauge in the fuel line and seems to be running at the right pressures 48-50psi with the vacuum hose disconnected 40psi with the vacuum hose connected There is about 500mm of hose between the fuel rail and the FPR. Obviously the FPR would be better directly off the fuel rail (as per factory). Could this be a possible issue? Should I be watching what the fuel pressure does at WOT? Quote Link to comment Share on other sites More sharing options...
AllTorque Posted October 15, 2014 Share Posted October 15, 2014 Have you checked your spark plugs and leads lately? 1 Quote Link to comment Share on other sites More sharing options...
Seedy Al Posted October 15, 2014 Share Posted October 15, 2014 I think you need to stop thinking about the fuel pressure reg and look else where. I have no idea what sort of components you're system runs. Quote Link to comment Share on other sites More sharing options...
Stu Posted October 15, 2014 Share Posted October 15, 2014 Had a wide band on it? Might have some crazy weird fueling going on Quote Link to comment Share on other sites More sharing options...
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