1995 Chevrolet Silverado Idle Issue

Tiny
KMIDDLE1957
  • MEMBER
  • 1995 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 150,000 MILES
1995 K1500, LO5 5.7, 150,000 miles.
Fires off at start then drops out. Does that 3 or 4 times then won't fire without some throttle. Is very rich, have to hold it at 2000rpm until it clears out, then it idles smooth @650. In a few minutes the idle increases to 1000 over about 30 seconds and stays there.
Heads are fresh. Exhaust with mid-length headers, true duels, cat delete is new. Cam, lifters and timing set are new. Cam is lightly upgraded;.420 gross lift, 112 lobe center, 120 separation. Do not recall the duration but know all specs are within the ECM's range of control.
O2, temp, EGR and valve, IAC are new. TB is clean, injectors rebuilt. Cap, rotor, module, wires and plugs are new. ECM is fresh.
There are no vacuum leaks to be found, replaced all of the tubing anyway. Static timing is at zero.
Fuel/air filters new. TB regulator delete, external AFPR is new, vacuum referenced and pressure set at 15psi. Have moved that up and down with little effect other than a lean stumble when set below @10psi. Had a 1” TB spacer, removed that with no effect.
A/C deleted, every thing under the hood.
Thermostat was a 180, changed to 160 with no effect. Engine reaches temp in a reasonable period of time.
Been driven 45-50 highway miles several times with no effect.
Fuel is 93 octane.
When the IAC is reset the truck idles ok a few minutes then climbs, does it every time.
MAP and TPS have not been changed, are on order. Both appear to be in spec but what the hell might as well invite them to the party too.
When driven codes 32 (IAC) set every time, 35 (EGR) set intermittent. SEL light would come and go.
Once while running in park after a IAC reset code 25 (Speed sensor) set.
All grounds are in place and intact, I've added a few with no effect.

All started with the exhaust. Knew it was shot and lucked into a deal with the headers. A previous owner had already dropped the cat. Ran all new 2 1/2” through an x-over can.
What I thought was a lifter tap or exhaust leak turned out to be a valve on #8, not enough to make it miss but did notice it was slightly down on power. Could not say with the iron manifolds, became obvious with the headrers. Thus the fresh heads. The rest is history.
Have I missed anything?
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Saturday, September 12th, 2015 AT 3:27 PM

3 Replies

Tiny
HMAC300
  • EXPERT
Remove iac and reset prior to driving. Then drive over 40 mph to reset iac. Fuel pressure should be 9-11 psi if a tbi unit also check under tbi for a blown gasket and soft hoses leading to map as it can suck together and map doesn't get any vacuum. Check egr passages for partial to clogged and clean out. Check intake vacuum if 15" or below suspect a clogged exhaust.
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Sunday, September 13th, 2015 AT 8:30 AM
Tiny
KMIDDLE1957
  • MEMBER
Bases covered already. No leaks, I replaced the lines with tubing no small I.C. Motor can collapse with vacuum and used clamps on every connection in the circuit. TB and intake manifold are clean, crossed my I's and dotted the T's while I had the top off the motor. Brake booster is good. IAC is new, have reset it a number of different ways, have driven at least 40 miles 3 times. Started out at the correct idle but was at 1000 when returned. Idles smooth at 650-700 in park, 600 in drive with brakes set. After idling a few minutes it apparently goes closed loop, the idle moves to 1000 and is running rich. Does this every time I've reset the IAC. No codes are being set during the event.
Have yet to change the MAP and TPS. Due to the cam change could it be the minimum throttle opening?
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Monday, September 14th, 2015 AT 9:09 AM
Tiny
HMAC300
  • EXPERT
It cold be fuel pressure a little to high, if you can adjust it try 12" also may be cam and what is vacuum on intake at idle?
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Monday, September 14th, 2015 AT 12:07 PM

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