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Bryan in Iowa
01-20-2005 19:40:21
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Re: Off topic, Chevy truck help! in reply to knauffj, 01-20-2005 12:21:32
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Well, where do we start ? Ok I'll take stab at this .I get lot of calls during the week like yours.Over the phone diagnoisis is lil tuff,,lol.
First thing , stop throwing parts at it ! Fuel pressure on these has to be exact, not oh its getting fuel , or yea it sprays out injector . Check fuel pressure key on engine off,write it down ! Watch pressure as it peaks,note how fast it falls off , and where it ends up . System should hold residule pressure after pump stops. If falls off quickly you need to pinch return line and retest . If pressure holds now, you have reg stuck open or leaking . If pressure still drops after pump stops, possible injector leaking, or bad in pump check valve , can check by pinching off supply line ahead of pump, if stable ,,bad pump. Also you should run fuel pump volume test. There should be a wire to energize fuel pump.Open supply line , energize pump,should just about fill pop bottle 3/4 full or close to it in 30 seconds .If low , check pump, filter , line restrictions ,etc..That pretty much is complete fuel pressure test . Make note of pressure when cranking , running , and when truck dies . Any changes indicate pump shutting off . But you should still be able to keep pump running by prime connector . Next thing I'd look at it engine vacuum. Should be steady , at least 12 inches, any less and you have either timing problem , timing chain jumped , ignition timing off, or valve spring problem , or plugged exhaust . Double check ignition timing,,MAKE SURE you unplug timing connector , either on firewall or under heater box on pass side in cab . Very important , if not unplugged timing can be off as much as 30 degrees . TO check back pressure either pull oxygen sensor and put gauge in there ,,or just drop pipes down from manifold 3/4 to an inch . If runs okay ,pluuged exhaust, converter etc . Also look at Egr,,if stuck open acts like big vacuum leak . Low vacuum will make map sensor fatten up fuel mixture , run pig rich . Okay if have good manifold vacuum , next I'd wanna see what Injector pulse width is . If you have diagnostic scanner that will show it , you should be seeing around 1-4 mil at idle . If it is way more than that computer is tryign to fatten up mixture 10.0 - 18.0 milsec is pretty rich. More likely vac problem still if that high . Also Map sensor voltage should be close to 1.2- 2 .0 volts at Idle . if up around 3 -5 v either map is bad , or reading low vacuum,,you can fool Map sensor with handheld vac pump, hook up to port put 20 inches to it , see what happens ? If runs ok,look at causes of low vacuum. Few other thing to check out .These trucks are famous for distributor shafts getting magnatised, makes em run like crap . Weak module , weak coil .
Sorry for the long rant , but these are great motors. You just need to have all your ducks in a row before you start slapping parts on em or asking a tech "whats wrong with it ?" with out having done some basic testing and results to go by . I see it in my shop all the time . Most of the time we get it fixed , few that bite us , but keep after them tiil they're back on the road . If you still having problems , shoot me an e=mail ,,can see what we missed or you can give me phone call . Good luck Bryan
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