Rough idle/ stalls. Map sensor

200

Asked by Chris_in_mt Oct 11, 2017 at 02:42 AM about the 1991 Chevrolet Blazer Silverado 2-Door 4WD

Question type: Maintenance & Repair

Ok,  I've hit a wall on this one.  Haven't had a chevy stump me like this yet.  Customer
brought this in,  complaint was sudden loss of power.  Ran like a top,  then fell on its face.
Engine would start,  come up to high idle for about 3 seconds, then die.  Stepping on
throttle made no difference.  Fathering throttle after getting it to start would get it to run
very rough but timing advance was obviously off.

  General look over,  needed new plugs,  cap and rotor.  Wires we're recently changed,  and
all ohm'd good.  Made no change in running condition.  Dig a little deeper into dist,  found
ign mod had rusty bolts,  connector plugs corroded,  mag pick up rusted but seemed ok.  
Replaced ign mod,  no change.  Replaced dist assembly,  had to cut wire to try to set base
timing,  wouldn't idle well enough to set it proper.  Moved on to map sensor,  unplugged
vacuum from sensor and plug vacuum to tbI and idle stabilized...  Momentarily.  Unplug
elec connector from MAP and idle went to poo again.  I've changed out EVERY elec
sensor now with no change in drivability. I've pinned out the ECM voltages against the
FSM values and I'm close enough to them that I don't see an issue from the minimal
difference.  We now have THREE test ECM's and now a new HYPERTECH prom chip.  

So heres my question.  The only thing I do not understand is my ohm readings at A12
(system ground at ecm)  is 4.5+/- ohms key off,  and 62-78 ohms KOEO.  Why the
change?  Same for D1 at ecm, and at D2 (MAP ground)  I go from 5 ohm key off to 101.5
ohm key on.  These readings are from ecm to bat neg post.

Also,  my MAP sensor voltage (C11) is only 4.08v key on.  FSM says should be 4.8v.  I've
had 3 map sensors on it and they all read the same,  and engine reacts the same when
vacuum is applied.  Runs until vacuum reaches 12",  and voltage is about 1.4v,  then more
vacuum/less voltage and its sputters and dies.  Engine holds a VERY steady 16" vacuum
at idle.  

Help!!!!

1 Answer

200

So I figured it out. Just so everyone else knows too.. I checked fuel pressure at the beginning of all this, too. Spec is 8-13 psi I believe. We had a solid 9, before filter, after filter, Also when dumping/releasing pressure at the gauge manifold. Found the EGR valve wouldn't hold vacuum, replaced it. Had the TBI rebuilt also. Customer wanted the fuel pump replaced, and we had put it off for a couple months. Today, I replaced the fuel pump, found that the hose connecting the pump to the steel line on the assembly had blown out. Original I'm guessing. Thick and spongy. Installed new pump, sock, and connecting hose. Got it all together and primed and fired it up. Took a minute for it to settle out, but now it idles. Finally. Throttle respond is good.

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