How to correct extreme idle speed on 1987 GMC R1500 with HEI and 5.0L engine?

5

Asked by Apr 27, 2015 at 02:59 PM about the 1988 GMC Sierra 1500 C1500 Standard Cab LB

Question type: Maintenance & Repair

Been doing this for over 35 years, never had this problem before.
Got truck cheap, No maintain history. Idles about 2000rpm. Have check for vacuum leaks, stalls when air is blocked. Base timing correct. When EST connection unplugged, idle speed near specs. Have replaced fuel pump/filter (pressures within specs), ESC module, overhauled TBI and ECU. Only thing left I think is distributor and/or ignition module.
Any ideas? Am too old to climb back there unless there is a good reason.
Thanks. Cliff

6 Answers

7,175

Were all of the sensors on the throttle body replaced?

Best Answer
7,175

Is the throttle stop tab bent, click on picture

1 out of 1 people think this is helpful.
5

Did not replace MAP. Tried short test. Disconnected and plugged MAP vacuum hose. Idle dropped below spec idle then returned to extreme idle speed. Connected vac hose and disconnected MAP electrical, got same results, dropped to spec idle and then returned to high idle. Forgot to mention that engine cranks slow like ignition timing is too far advanced but does the same with the ESC spout connected/disconnected. Throttle stop tab not bent and screwing the stop screw out does nothing. Throttle plates in TBI are closed completely. This usually points to vacuum leak but tested fine there plus it would be one heck of a leak..

7,175

Have you scanned it yet?

5

First thing. no DTCs. What data is available looks close to specs exc for rpm and timing. Tomorrow I am going back to basics: Manual check of timing, rotor position, wire position. Maybe, Maybe, some one pulled dist/cap and maybe misaligned rotor (prob not, fits one way) and plug wires then tried to correct their mistake by juggling wires to at least make engine run . Electronics no good if mechanics are no good. will repost tomorrow evening. Thanks for you ideas so far.

5

Sorry took so long to get back. After checking everything I could think of, decided to go ahead and replace intake manifold gasket. Had to force dist out of intake with crowbar and chisels (last removed was during engine assembly in 1987). Needless to say, destroyed the dist. Got new one and installed the intake gaskets. But "may have" found the problem in the process. Take a look at the photo of the dist pu coil and reluctor. completely rusted. the reluctor was dragging on the pu coil's every tooth, not just touching but actually dragging across the individual teeth. Don't know for sure but this is the only thing that is oblivious.

Your Answer

Add photo

Related Questions

  • rough idle, stalling, loss of power. 225 answers

    o2 sensor code 44 (lean) Replaced: coolant temp sensor, tps, map sensor, fuel filter, and o2 sensor. It will run normal on occasion but only lasts about 45 seconds or so. I don't think its the torqu...

    1995 GMC Sierra 1500 K1500 SL 4WD Extended Cab Stepside SB

    -

    Maintenance & Repair

  • Engine swap out 2 answers

    I have a 98 Ford explorer 5.0L engine. Will it fit in 98 GMC Silverado 1500

    1998 GMC Sierra 1500

    -

    Maintenance & Repair

  • 1990 GMC Sierra low idle and stalling 3 answers

    I have a 1990 k1500 low idle and stalling problem, I replaced my fuel filter and spark plugs, i changed the fuel pump 3 years ago. Weird thing is when I rev the engine the oil display moves with ...

    1990 GMC Sierra 1500 K1500 4WD Standard Cab LB

    -

    Maintenance & Repair

  • I NEED A REAL GURU ON THIS ONE!!! 1993 GMC K1500 5.0L Have changed most of the fuel system but still having issues of not wanting to start. 4 answers

    The truck is a standard cab with an 8ft bed and 131" wheel base if that makes any difference. So here is the scenario: the truck was having issues of starting and then would start to stall and I wou...

    1993 GMC Sierra 1500 K1500 4WD Standard Cab LB

    -

    Maintenance & Repair

  • engine idles up and down 2 answers

    when crank my 1989 gmc 1500 350 engine up it idles up and down like maybe the timing is off but not so. When i take off it suptters but do does backfire. IT acts like it is starving for fuel. W...

    1989 GMC Sierra 1500 C1500 Standard Cab SB

    -

    General

Sierra 1500

Looking for a Used Sierra 1500 in your area?

CarGurus has 64,952 nationwide Sierra 1500 listings starting at $1,500.

ZIP:

Search GMC Sierra 1500 Questions

GMC Sierra 1500 Experts

#1 dragonflyoffshore
dragonflyoffshore
Reputation 4,850
#2 John Saffrahn
John Saffrahn
Reputation 3,420
#3 Glenn Clingenpeel
Glenn Clingenpeel
Reputation 3,020
View All

Related Models For Sale

Used Ford F-150
2,414 Great Deals out of 210,626 listings starting at $895
Used Chevrolet Camaro
143 Great Deals out of 35,667 listings starting at $1,995
Used GMC Sierra 2500
4 Great Deals out of 163 listings starting at $2,400

Used Cars For Sale

2017 GMC Sierra 1500 For Sale
34 Great Deals out of 38,982 listings starting at $21,410
2016 GMC Sierra 1500 For Sale
91 Great Deals out of 3,829 listings starting at $18,810
2015 GMC Sierra 1500 For Sale
127 Great Deals out of 4,091 listings starting at $14,999
2014 GMC Sierra 1500 For Sale
139 Great Deals out of 4,453 listings starting at $14,495
2013 GMC Sierra 1500 For Sale
72 Great Deals out of 2,310 listings starting at $8,096