Significant Success, TBI Idle Hunting and Idle Miss
 

In case it may help others. I've taken a deliberative approach as the 87 Jimmy (350/700R4, TBI, 94K original miles) is driven 700 miles weekly as a daily driver. Exhaust is true dual, 2.5" with no cat and with two irritatingly loud magnaflows.

All original and good mechanical condition but with warm (restart only, and disappears as soon as you drive a few yards) idle hunting and an occasional warm idle miss. Figured I'd start with cooling and move through all the systems since they are 30 years old. No reason not to freshen it up using all AC parts for reliability.

Initial fuel mileage on 87 Octane was about 14 mpg.

Since the radiator was leaking, new rad, coolant, coolant temp sensor and temp sending unit and new stant 195 thermostat. No change in idle issues.

Plugs were rusty and worn variously from 50 thou to 90 thou, wires new so new NGK V power gapped 45 thou, new coil, new brass cap, new rotor and reset warm timing, wire disconnected, from 10 After to 0. Much more power, No change in idle issues.

Rebuilt TBI with new Standard Motor injectors, 14 lb regulator spring, 1/4 injector spacer, new air cleaner, new IAC, new AC Fuel filter, new tank, new pump. Fuel mileage now averaging 15.9 mpg and no change in idle issues.

New AC Professional EGR and checked solenoid, no issues. But, no change in idle issues.

Checked for vacuum leaks. None. Checked intake manifold and TBI, no leaks (Vacuum or Fuel). Replaced every inch of vacuum hose. Subsequently checked vacuum at every TBI port.....20" steady with a slow and even wander from 19" to 20" on those occasions where the idle would wander slightly after a warm restart. No change in idle issues.

Looked at the tag on the radiator support.....plug gap is listed as 35 thou. Remembering I had a new AC factory off cap coil, decided to give it a try as I also remembered the original AC plugs were worn from wide to very wide. Pulled the new plugs, all had the proper toast brown steaks and the temp line was on the flat just before the turn of the electrode. No wear and still sitting at a snug 45 thou gap. So, they looked great after 2000 miles of driving. Regapped them all to 35 thou and reinstalled.

Driving yesterday and today.....the occasional warm idle miss is completely gone. The warm restart wander is significantly reduced. I suspect I'll continue to update the old sensors anyway, after all they are 30. Next on the list will be egr solenoid and MAP sensor (both checked good though old). Then the O2 sensor, TPS, ESC and finally a new dizzy and ignition module.

Progress.

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Thanks for sharing. I have TBI and I’ve noticed my idle wanders a little sometimes after it’s warmed up.

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Still trying to nail that wander issue down but its for sure, my lesson learned to date would be to gap plugs to factory specification where the ignition (coil, dizzy, voltage, etc) is still all factory specification. Wish I'da done that up front!

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

My truck is kind of a hodge podge of parts. I bought an 87’ 4x4 that someone else had started restoring with an 83’ cab because the 87’ cab was rusted out. Then I bought an 89’ Suburban for parts. I ended up using the wire harness, computer etc. from the Suburban. The Suburban ran perfect before I tore it apart. I’ve got the truck running pretty good, put I still think something is a little off. When it gets warm, the idle wanders a little. Every once in a while it will even die when I go from park to reverse. Overall it runs pretty good, but I would love to get it running even better like it should be.

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Just an update, 400 miles down the road. With gapping the plugs at 35 thou, the random idle miss is cured. As for the wandering idle, I have not noticed it but once in the last several days and that only slightly one time. EGR solenoid and MAP sensor arrived. Should get them in this weekend.

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Evening update. installed new MAP and new EGR solenoid. Did a hot restart from about 150+ degrees and let it idle up to full temp with the thermostat open.....no wander in the idle. Shut it down for 5 minutes just like pulling into the gas station for smokes and a coffee, hot restart from 200+ degrees and no wandering idle. Also noted on both restarts the engine rpm responds more rapidly, cycling off the fast idle and down to a smooth hot idle right quickly now, not the slow slow drop in rpm it usually exhibited before. 3 more days to work this week so that's 420 miles on test runs. should know if I nailed the wandering idle issue by then. i'll let you know.


maybe soon I can move on to a couple split flanges and snug up that pesky leaking PS side manifold doughnut......

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

here are the recent fuel mileage numbers for jan and feb.

All are 87 octane kangaroo and pilot juice with ethanol.

all are country roads averaging 55-60 mph with some 70 mph highway time.

all but one are two wheel drive (14.57 mpg is the day spent in 4wd)

286 miles, 17.91 gal, 15.97 mpg 16 jan 18

254 miles, 17.3 gal, 14.75 mpg jan 18

328 miles, 20.0 gal, 16.4 mpg, 24 jan 18

292 miles, 18.4 gal, 15.86 mpg, 27 jan 18

335 miles, 21.69 gal, 15.43 mpg, 31 jan 18

316 miles, 19.17 gal, 16.54 mpg, 2 feb 18

300 miles, 19.5 gal, 15.38 mpg, 9 feb 18

352 miles, 22.25 gal, 15.82 mpg, 14 feb 18

333 miles, 22.2 gal, 15 mpg, 17 feb 18

389 miles, 23.1 gal, 16.83 mpg, 21 feb 18

323 miles, 20.5 gal, 15.75 mpg, 26 feb 18

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

A setback....but humorous. Started out to work this am with the new MAP and EGR solenoid in place. Pullin like a freight train, no idle wander.

Topped off with 87 octane and read 17.56 mpg. Happy.

Rolled out. Hit 60 mph.......SES lights up.

Get to work and pulled Codes 32 33 and 42 with a paperclip. Darn. EGR and MAP

Frustrated. Locked keys in truck (1.5 hr ride for wife to bring spares) but a buddy had tools and training and a book and zip zoom door open and keys recovered. So. No angry wife. No paint damage from a coat hanger neither.

Checked connectors on sensors, cleared codes and drove home.

Truck pulling like a train and SES pops on. Damn.

Get home. Install the old MAP and DAMN!!!!

Last night I forgot to reinstall the EGR solenoid wiring harness.

Plugged it in. Put the new MAP back on. Cleared codes. Idles perfect.

I'm guessing tomorrow's ride is a bit less likely to flip the SES on!

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Morning ride, 70 miles. NO SES!

I suppose you do actually have to attach all the wires under the hood in the right places......

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Great write up and thanks for posting it up. Sometimes you forget it can be more entertaining to see one brought back into OEM spec than throwing aftermarket parts at it.

Re: Significant Success, TBI Idle Hunting and Idle Miss
 

Thanks. I think most of us get into sensor hell from time to time. I have some more to replace, the goal is to get to all of them by spring....one system at a time with driving between (since its a daily driver and to ensure each change is a known quantity before proceeding). Mostly, its fun, kind of.....maybe.....well....?