[Diy_efi] desperate need of help

Shannen Durphey shannen at grolen.com
Wed May 28 03:29:31 GMT 2003


Typically when the mechanical parts are all ok, open loop idle is the 
easiest and fastest cure for this symptom.

Depending on how much fluctuation the map sensor displays, the problem 
may be aggravated by spark timing changing.  A good test might be 
reading the timing with a timing light while the idle speed is changing 
to get an idea how much advance is present, then adjusting the 
distributor to provide similar base timing with the bypass disconnected. 
  If the idle is smooth without ecm control of timing, some additional 
chip work might help.

If the engine is EGR equipped ensure that the valve is closed and no 
vacuum is present at idle.  If the engine is AIR pump equipped, ensure 
the pump is not active.  Be sure there are no exhaust leaks.  If all 
seems well, look for solutions related to tuning.  While the aftermarket 
company may not be able to find answers to your problem, they should at 
least be able to help you figure out what the correct questions are.

Shannen

Michael Opp wrote:
> I have a 1991 k2500 with a 350 t.b.i. motor.  I have
> recently rebuilt the engine and decided to upgrade.  I
> installed edelbrock aluminum centerbolt heads with an
> aluminum performer intake, with an adapter plate to
> use my t.b.i.  I installed a comp cams oe roller with
> 206 degrees int. and 212 degrees of ex. duration at
> .050 lift.  The lobe lift is .320in and .325 ex. the
> cam is ground on 112 lobe seperation.  I had a custom
> prom made by an aftermarket company.  I have spoken to
> them many times and cannot get any answers for my
> problem.  The engine idles pretty good in open loop. 
> Once in closed loop the idle will not stabilize.  It
> will surge up and down about 200 rpm.  I have checked
> for a vacuum leak and have none.  The block learns are
> around 114 at idle and map kpa is about 44.  The iac
> counts stay steady around 20 to 24 counts.  Under no
> load in park while holding the accelerator steady
> about 1500rpm the rpm will flucuate up and down about
> 100 rpm.  The engine runs well under load.  I have
> checked the usual things like compression, valve adj.
> vacuum is good at around 21 in hg.  I spent time
> drifting propane around all vacuum fittings and into
> the crankcase to check for a leak under the intake. I
> can't find one.  I need help to determine what the
> problem is and how I can go about fixing it.  I wound
> appreciate any input.
> 
> Thanks,
> Mike Opp
> 
> 
> =====


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