Greetings to all Chevy truck owners. Here is an interesting problem I am having with my 1987 Suburban. Ready for THIS engine issue?? I can hook up a Snap On MT2500 scanner to the ALDL plug and when I plug in the 12 volt accessory plug to power the scanner, the idle speed will S L O W L Y increase about 150 RPM. When I shift into ANY gear range (NOT Park or Neutral) and hold the brake, the rpm starts to increase more and I can hear the IAC opening up, ONLY WHEN THE SCANNER 12 lead is plugged in). I worked on these early systems and have the factory Chevrolet manuals on it with the wiring diagrams and it has kicked my butt!! I tried the other 12 volt lead I have with a gator clip directly to the battery positive, no change. My original problem was the old IAC was not working right and changed it with a new one (Standard Ignition Part Brand). My vacuum is at 22 inches and STEADY as a rock. I tried a second IAC and still no change. The EGR was changed last year and I have disconnected all the sensors and it STILL does it. I put the scanner on another vehicle, a 1994 Silverado and the scanner works flawlessly. ANYTIME I EVER had a problem with a scanner (The old TECH 1's) on the early TBI units doing this was a ground wire somewhere on the engine (Two on the front of the engine alternator bracket and two on the right rear of the cylinder head) either cut, loose or corroded. Nope, I found them ALL and they are nice and clean and tight, and even run extra ground wires to the body, frame AND engine...still NO CHANGE. BUT, the one strange issue I found was the black/white stripe wire in the back of the ALDL connector that goes to ENGINE GROUND is reading 56.7 ohms using a Fluke 77 DVOM meter. Ok, so just run an extra ground wire piggybacked to make sure it's grounded and it should work.....right?? NOPE, still does it. I do NOT like putting on parts to just (Try and see if it works), so I really don't have too many options left. I fully restored the vehicle so the fuel pump, pump relay, EGR, EGR solenoid, battery, cables, even an ECM was changed and STILL it has issues AND there are NO trouble codes. I know with as many wiring harnesses I have repaired/replaced in 30 years on the workroom floor as an automotive technician, this sounds electrical..........I still say a ground problem but what in the world have I overlooked??? I got so aggravated after pulling the dash out again that I just closed the garage door said "Another day ole' Mr. Burb".
CK5
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1987 Chevrolet Suburban Silverado 4X4 V10 5.7 TBI
- Thread starter Heckler
- Start date
I honestly can't remember, do these kick up the idle RPM with the scanner hooked up? I know TPI the same vintage does, but I don't remember on TBI.
IAC counts, TPS voltage, and timing would be what I'd take a look at when it's starting and as the idle creeps up.
Have you done the minimum air adjustment? Only once has that done anything for me, but that one time, it fixed the idle issue I was having.
IAC counts, TPS voltage, and timing would be what I'd take a look at when it's starting and as the idle creeps up.
Have you done the minimum air adjustment? Only once has that done anything for me, but that one time, it fixed the idle issue I was having.
Greetings and GOOD afternoon dyeager535!! Thanks for the response........ok........the timing is good, not bouncing around either. The BASE idle, according to the GM factory manual with the IAC fully seated, ground your ALDL with the famous two pronged key and then the IAC is disconnected and engine re-started and is dead on 550 RPM and as SMOOTH as a kitten. The 700R4 transmission is (LOOKS LIKE according to the 1987 code on the transmission case) original, rebuilt one time or another and it is the early 700R4 WITH OUT the Aux Valve Body. I even disconnected the transmission electrical plug, YES, I noticed with the pan off there is one pressure switch and it was evidently upgraded for a simple TCC circuit. Yes, the TCC does apply when driving it so I know it does work but even the plug unplugged, scanner hooked up it STILL does not perform properly. I am baffled but will clear my head and keep trying......thanks for the input, I REALLY appreciate it!!!
rampage
3/4 ton status
Ever hear of the ‘observer effect’? 
And the TPS is at .66 AT IDLE and the IAC will reset to 145 after key off then on again and when re starting, it drops back down...........untill you plug in the 12 volt power lead. And.......you can unplug the 12 volt power cord and physically hear the idle drop down just a little bit. I wonder........it's a longshot......but......I worked on MANY VW busses and beetles and Opel Kadette's and GT's and I know that the VW's would NOT charge the battery properly unless you used the German Bosch voltage regulator, the Standard Ignition (OR called standard motor parts) regulators would work about 6 days and then all of a sudden, the generator light would start flickering..........
Well, I have observed looking at it.........................but no, not really. At this point, I will try anything........................................LOL!Ever hear of the ‘observer effect’?
rampage
3/4 ton status
Basically it says that you can’t measure anything without affecting the system ... which is exactly what you’re doing.
It’s a joke son.
On your ALDL logs, are you getting a low voltage flag? I don't have an ALDL truck anymore, but it says something like "low voltage/IAC inhibited" Or, just check the voltage that's supplying the ECM. If it's less than 13 or so with the engine running, try a wire directly to the ECM (fused of course) to ensure it's getting enough voltage. If you do have good voltage at the ECM, I'm of no use to you...that's my only suggestion. Good luck!
Greetings anwat and thanks for the suggestion. I'm sorry but the pre OBD2 system on the '87 TBI doesn't have the smarts to list that parameter. I will do a voltage feed check on the ECM connector for sure. I am still running over all the possibilities................thanks again!!!
I had it on my 90 in as a flag...but the low voltage was making it do all sorts of odd things, one of which was changing the IAC for no apparent reason while idling. I never figured out where the voltage was leaking off, it was 13.7 coming out of the fuse block, but by the time it got to the ecm, it was down to 12.5. corroded wire? who knows, but it did make difference when it was fixed.
Hey THANKS anwat!! I looked further in the back of the Snap On MT2500 manual and in the very back of the book, it covers the ALDL wiring and how to test a couple of things. Does anyone know if the MT2500 scanner when used on the 1987 350 TBI system, uses the 12 volt positive lead ONLY? There is NOT a ground at all on the power lead in the side of the ALDL plug in connector (Gator clip is red for 12 volt positive) .......BUT.............I see on eBay that there are TWO different 12 volt hook up leads, one WITH the cigarette lighter plug (IT has the ground connection I think) and the other with one wire going to a red gator clip, 12 volt positive only. I wonder if the 12 volt side plug has to be the one with a ground on it?? I do NOT remember......................................
I've never used a scan tool, I just hook it from the car up to a serial port on a laptop and use WinALDL...But I do know there should be a ground connected along with the 12v from the battery...and I know it needs to be a battery lead, not an switched ignition hot. Maybe these will help with the pinout on the scan tool you're using...
Hey THANKS!! I am still working on it and will make sure all my ECM pins and connections are good. Time will tell.
black dawg
1/2 ton status
If you use the data stream portion of that scan tool, tbi motors do all kinds of weird stuff. Have to use the road test codes and data section. I dont have one in front of me, but I think that is what the section is called that you have to use. Otherwise, you will chase your tail trying to figure out why the idle is soo fast and the timing is all jacked up.
When I plug in my snapon scanner to my 89 v10 Blazer and plug in the power to the cig lighter port, my idle goes to exactly 1000rpms. I have to tell it diagnostic test drive for the idle to drop to normal...
