Re: Off topic, Chevy truck help!

Posted by Bryan in Iowa on January 20, 2005 at 19:40:21 from (64.12.117.7):

In Reply to: Off topic, Chevy truck help! posted by knauffj on January 20, 2005 at 12:21:32:

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

Follow Ups:

• Re: Off topic, Chevy truck help! joe e-tx 07:38:01 01/21/05 (1)
  • Re: Off topic, Chevy truck help! Bryan in Iowa 07:49:40 01/21/05 (0)

Post a Followup Name: E-Mail: Subject: 65534p> Comments: Optional Link URL: Link Title: Optional Image URL: Email Notification: If you check this box, email will be sent to you whenever someone replies to this message. Your email address must be entered above to receive notification. This notification will be cancelled automatically after 2 weeks.

TRACTOR PARTS	TRACTOR MANUALS

We sell tractor parts!  We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today. [ About Us ]