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Comment by Kevin Lee on July 12, 2009 at 7:44pm
Forgive me, I forgot one other thing. You said the misfire happens just off idle, part throttle. Try unplugging the EGR valve, plugging the vacuum hose to it. If it goes away, the valve is opening too soon. The cause for that will be either a weak valve, or partially clogged exhaust, causing too much backpressure
Comment by Kevin Lee on July 12, 2009 at 7:38pm
OK, Thomas, that means it is a throttle body engine. Going by what you have already done, I will say there are a couple possibilities left. One, you have a vacuum leak, like a bad hose, or even around the intake manifold. I use either a can of brake cleaner or carb cleaner to spray around and see if you make a change. If not, the problem most likely is engine internal. To pnpoint the cylinder, have a friend put it in gear and LIGHTLY brake torque the engin until the misfire happens. while holding it there, you take well insulated pliers (they do make spark plug wire pullers for this) and take one plug wire off at a time. The cylinder that doesnt change when taking off the wire is the problem cylinder. The next step would be to check static compression on that cylinder. With the tester, it should be around 120 to 150 PSI. Under 100 PSI, you have a weak cylinder for sure. You can also "read" the plug from the misfiring cylinder. If it is very white, no deposits at all, it is either lean on that cylinder, (the vacuum leak) or even possibly getting coolant into the cylinder, effectively "steaming" the plug clean. If it is black, the cylinder is weak, and cant effectively burn the fuel. If it is oily, have a lot of flaky deposits, you have engine problems for sure, like faulty valve guides, or even bad psiton rings. Hopefully these suggestions get you on the right track
Comment by THOMAS E EVERAGE JR. on July 12, 2009 at 7:26pm
1993
Comment by Kevin Lee on July 12, 2009 at 7:21pm
Thomas,
One, what year is your Suburban? Secondly, unplugging the MAP (Manifold Absolute Pressure) sensor will cause the computer to default to open loop status, and fatten up the fuel mixture.That is covering up the misfire, giving you false diagnosis. That is also why it dies when the A/C is on. I can give you a lot more ideas, but need to know what year it is so I know the setup.
Comment by David Foster on July 12, 2009 at 4:15pm
thanks, i will keep that in mind. the engine is due for a rebuild soon. it has 345,000km on it so when its rebuilt im adding some goodies to it
Comment by Kevin Lee on July 12, 2009 at 2:35am
Good to hear that the intake isnt leaking. If it should though, use Fel-Pro gaskets. They came up with a superior design over te stock ones. Just make sure the intake isnt pitted too bad around the coolant ports, and it should be fine after that. Just some info if you will ever need it
Comment by David Foster on July 11, 2009 at 8:15pm
ended up cutting the tube to get it out of the intake
manifold
Comment by David Foster on July 11, 2009 at 8:12pm
thats it exactly! intake gasket luckily seems to be good
Comment by Kevin Lee on July 11, 2009 at 5:39pm
I know exactly what you are talking about. I have torn many of those apart for intake gaskets leaking. It comes off the left exhaust manifold, up the back and follows the intake where it goes into the left front
Comment by David Foster on July 11, 2009 at 5:11pm
The headers are finally in!. it took a little more work then i had anticipated but the end results are very satisfying. Thank you to everyone who offered me there advice in removing the EGR fitting from the exhaust manifold i tried quite a few of your suggestions but to no avail, until i went to the local Chevy dealership to find out what i needed to do to get the fitting out. and to order a new EGR tube incase i broke the old one..... turns out the EGR tube and fitting are made out of Aluminium and it was put in to a cast iorn manifold that would be exposed to lots of heat.. so needless to say those 2 pieces are now a very solid one. to any one wondering what im talking about this was on a 1996 VORTEC 350
 

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