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Old 06-10-2010, 06:55 PM
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4.3L v-6.. diesel problem

hay guys i cant seem to get this engine right. i know my parts are part of the problem, but man... im strugglin here.

i cant get this engine to not diesel!
the application is...
1991 4.3L v-6 160k miles.
non ballance shaft engine
4 barrel intake with a holley 500 cfm 2 bbl
single points distributor, with new points (@.016"), condensor, and coil.
the floats are adjusted at the bottom of the sight window
idle screws are set to maximum vacuum.

timing is at 14 degrees, but does it @ 8 deg, and at 22 degrees also!

whats my problem?

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Old 06-10-2010, 09:29 PM
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slow the engine down and it wont diesel

the IAC is supposed to extend when shut down and cause engine to stall correctly withotu trying to run on

is the IAC working?

when you rev it up can you see the pintle moving in and out to control air and idle speed?

remov eit and clean the carbon off it and out of the hole in the TB unit too.

set it to about 1 and 1/8" from mounting surface to the end of the pintle then put it back in
dont over tighten, just go a little snug with it or youll bust it off.,.,., or try another IAc off another good running late 1980s and 1990s TBI engine any size even a 1.8L or 2.0L or v-8 or 2.5L etc., all same., car truck or van,.

got a vacuum leak? spray cleaner around intake gaskets and tb unit and hoses etc, make sure no speed up happens when you spray anywhere....

vacuum reading should be around 15 to 18 at idle

if nto then timing may be off

if its retarded too much then unplug the knock sensor and see.,

it may be over compensating for something failed,

temp sensor also if its reading too low then it may be trying to run rich mixture thinking the engine is oclder than it actually is

does the engine start right up very quick no drag? do yuo ever hear a little pinging or rattling inside the engine when accelerating under lod when hot or warm?

if either thing or both things then timing is way advanced, need to back it off some and this would slow idle speed down also.... i would try turning distrib housing clockwise while running to slow speed down some, and make sure still runs and starts up great, if so then that should do it, the ECm is supposed to control timing changes etc from wherever the base timiung is set(base distributor position)

its gotta be vacuum leak and or timing issue if not issue with IAC also make sure pcv hose connected properly and good valve on it with working parts in it, if its stuck open or too easily free flowing it will raise the idle speed quite a bit, they are sensitive about too much air getting into the tb unit,,,, too much of it and they will run fast,,,,, i own and drive many tbi engines these years including my daily river 91 s-10 4.3L Z and have found out all about these quirks on these engines...... ive modified mine with raised roof FP regulator, split duals and larger y pipe, and a few other things and it runs crazy strong awesome, and 202k miles on it now going strong., i routinely remove and clean the iac and have diasbled the egr valve that commonly stick open, and repaired the pcv hose 90 degree boot that commonly clog up on these, and new cap and rotor every couple months that always burn up on these tbi engines, and other things,. removing the egr valve parts and gutting out my pcv valve makes mine idle fast but i like it and i always shut it off in drive, so no dieseling issues. i purposely want mine to run 800 rpm idle. but it still doesnt diesel even when shut off in park or neutral. i think my iac is woring very well at cutting off the idle air at shut down time., so it wont try to run on,.

Last edited by fast68; 06-10-2010 at 09:41 PM.
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Old 06-10-2010, 10:51 PM
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First off, throw the points ignition away, the best place for points is the trash. The timing should be 12 degrees base with vacuum advance disconnected and plugged, reconnect vac advance to a full time manifold vacuum source or one that pulls vacuum at idle, with vac advance plugged in you should have 20-24 degrees base timing total. If the problem continues, shine a light in the carbs throat at idle, if you see fuel coming out of the boosters, either the float is too high, the needle is stuck open off the seat, or the fuel pressure is too high. You may also want to check the balancer, 4.3 balancers are known to slip which will knock your timing reading off. Lastly, check it for vacuum leaks.
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