JamesHigdon Posted March 13, 2012 Report Share Posted March 13, 2012 We have a '93 C1500 with a mild TBI 355 that has a series of odd problems and one of the possible causes is a leaking intake manifold gasket. The motor is running Patriot Vortec heads and a Holley TBI intake with the block decked just a little for cleanup. We are chasing a high-idle/high-fuel consumption issue and are beginning to think it may run back to an intake manifold leak at the bottom of the intake runners into the valley, the issue is I know no way to confirm or disprove that? I know a vac. leak into the atmosphere will cause those two conditions exactly on a TBI motor but I am not sure if a leak into the valley would? The customer stated the fit between the intake and the heads was a little "off" as the manifold dropped down in and had some concern himself about an intake leak at the valley. The parts where all new when installed and the customer used the proper paper/silicon gasket (equivalent of a felpro 1205) during assembly. Link to comment Share on other sites More sharing options...
Alloutpc17 Posted March 13, 2012 Report Share Posted March 13, 2012 If the intake was leaking at the base you would see some oil or water leaking out also. Link to comment Share on other sites More sharing options...
KISS_Racing Posted March 13, 2012 Report Share Posted March 13, 2012 Just curious who did this? "Holley TBI intake with the block decked just a little for cleanup. " I have seen several coming from one specific machine shop being off to the point a friend made the machine shop rebuild his whole motor. If you would prefer to PM me even better. Also, do you notice any crankcase blowby? Link to comment Share on other sites More sharing options...
PAPA Posted March 13, 2012 Report Share Posted March 13, 2012 James Check your PM's Link to comment Share on other sites More sharing options...
JamesHigdon Posted March 13, 2012 Author Report Share Posted March 13, 2012 Just curious who did this? "Holley TBI intake with the block decked just a little for cleanup. " I have seen several coming from one specific machine shop being off to the point a friend made the machine shop rebuild his whole motor. If you would prefer to PM me even better. Also, do you notice any crankcase blowby? Well, it is leaking at the upper intake at this point, which the owner swore it wasn't but we have a starting point at least. The gaskets don't appear to be sitting right down in the valley so we've got a few things to look at on that end...I will PM you about the machine shop. Thanks PAPA, I'll look into that as well but up to this point all data recording has shown no issues in that vein. Link to comment Share on other sites More sharing options...
KISS_Racing Posted March 13, 2012 Report Share Posted March 13, 2012 Read on another forum a guy had to use Liquid Copper to get a good set of Felpro gaskets to seal... I'd almost say you may have to R&R the intake paying special attention to checking it without the gaskets. Also put a straight edge on every flat surface. Also I'd check your angle and reverse angle to make sure they are correct. Also, look for any scratches... I'd go with Liquid Copper if angles match up. Link to comment Share on other sites More sharing options...
jwmbishop Posted March 13, 2012 Report Share Posted March 13, 2012 If it has any hours on it the best indicator will be the port wash patterns. I chased a gen II that kept burning #2 exhaust - spotted the trail of black coming from under a valve cover bolt (after I made him bring the thing in un cleaned). He didn't hit the bolt hole porting - but he had removed a broken stud and did a helicoil - opened the hole to the port with the tap! The very slight leak was not visible without serious magnification so I missed it - TWICE. Link to comment Share on other sites More sharing options...
texasprd Posted June 8, 2012 Report Share Posted June 8, 2012 We have a '93 C1500 with a mild TBI 355 that has a series of odd problems and one of the possible causes is a leaking intake manifold gasket. The motor is running Patriot Vortec heads and a Holley TBI intake with the block decked just a little for cleanup. We are chasing a high-idle/high-fuel consumption issue and are beginning to think it may run back to an intake manifold leak at the bottom of the intake runners into the valley, the issue is I know no way to confirm or disprove that? I know a vac. leak into the atmosphere will cause those two conditions exactly on a TBI motor but I am not sure if a leak into the valley would? The customer stated the fit between the intake and the heads was a little "off" as the manifold dropped down in and had some concern himself about an intake leak at the valley. The parts where all new when installed and the customer used the proper paper/silicon gasket (equivalent of a felpro 1205) during assembly. Got a propane torch? Bleed propane through the oil hole or breather in the valve cover and see if the idle quality/speed changes. If there's a vacuum leak at the bottom of the ports, it'll suck the propane into the cylinders. This technique is usually used for testing O2 sensor response to find a biased or sluggish sensor, but it seems like it could work here, as well. Google for "oxygen sensor propane test" Link to comment Share on other sites More sharing options...
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