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That's not quite true, the Minimum Idle Stop screw should always hold the butterflies open (unless someone messed with it). When you set Min Idle, you close the IAC all the way, then adjust it.plug the IAC port on the back left side of the throttle body to check for vacuum leaks, truck shouldn’t run or start if there’s no vacuum leaks.
Any ideas why the truck ran so good for a bit with knock sensor disconnecEd then ran like crap again after 10 minutes of it disconnected?
Hey im new here, i have a 1988 K5 Blazer tbi (i know not gmt400 but you guys know a lot about tbi) Ive been having this issue where the truck will run super rich causing it to be very sluggish and guzzle gas. There is no smoke out the tailpipe but i went from averaging over 15 down to getting under 5mpg. Ive replaced the o2, cts and connector, map connector, iac and connector, tps connector and tested tps to verify is correct, rebuild throttle body, new fuel pump reg. and filter, new air cleaner, cleaned both grounds on top of manifold, new egr, almost everything. The truck also has developed a rough high idle and jumps from 700-800rpm then drops to jumping 600-700rpm. Please help Im lost herita
Check your vacuum advance this happened to me on my 91 many years ago. It lost all its power just dumping fuel in.. it's to the right side behind the tbi and has a little ufo shaped piece on it.. also look up symptoms for vacuum advance not working sounds like the issue. But I'm not sure worth a lookI’ve used a multimeter to test the other sensors and they came back good backblading and testing resistance, the whole issue is a don’t have a scan tool and I’m not even sure what to look for, I’ve tried looking it up but all I can find is those little obd1 ”scanners” that work the same as a paper clip in the diagnostic terminals.
Any ideas why the truck ran so good for a bit with knock sensor disconnecEd then ran like crap again after 10 minutes of it disconnected
So do you think it’s the knock sensor being bad, so when I popped it off it was how it should be then the computer got a little freaked out with no signal causing it to drop timing. Would that mean a new knock sensor might fix my issue maybe??? The check engine light didn’t come on with it disconnected, but I do know it is functional because it is on with the key on but engine off, but it shuts immediately when engine turns overYes. Your symptoms would indicate that when you first disconnected the sensor, the full timing
was available and your engine ran well. But during a subsequent ongoing ECM diagnostic check
run while you are driving (ie: the ECM advances the timing on purpose until it gets the expected
knock signal from the KS) ...the lack of expected signal from the (disconnected) sensor causes
the ECM to go into extra-cautious (self-protect) ignition timing mode.
If this happens, I would expect that when this happens that the associated DTC code is stored
in the ECM and that the SES light is illuminated? By the way, is the SES light illuminating at KOEO
and then extinguishing shortly after the engine starts?
There's a pretty good description of all this in the FSM. And the ECM diagnostic seems to be
pretty good at ferreting out a weak/completely broke Knock Sensor...but at the same time the
algorithm is not so good at figuring out that a Knock Sensor has become too sensitive in it's
old age and allows a bad sensor to overly retard the ignition timing, causing the poor driveability
like what you are experiencing.
Again, any input on stored codes and SES light behavior would be a big help.
Q: Does the above make sense?
The grounds I believe were good for a while, then the issues started once I started replacing sensors as preventative, but I quickly fixed up the grounds good after doing cts connector just in case. It’s mostly been things I replace and it runs good for a week then a new problem arises (really really tiring to deal with) but when I bought the truck just about every single part was factory original, even the shocks were from factory. So I think it’s more so old parts worn out rather than ECM but my whole issue is since I don’t have a scan tool it’s harder to test if it’s new parts that are faulty, or old parts that are well... OldDon't chase after the timing chain,. unless its worn a hole in the cover it'll run fine sbc pretty bullet proof,.
use a strong LED light to look at injector spray
I personally think u may have an ecm problem,. after the evm works with a deteriorating ground for a couple of years,. it can sometimes give out,.
i have plugged in all kinds of ecm's not for the vehicle or engine size i have been playing with,. and most TBI ecms will fire up the motor and run ok, trans shift are often off tho,.
GMT 400 trucks do not have a vacuum advance. Unless you installed an earlier style distributor your '91 or it is a different make the vacuum advance was not your problemCheck your vacuum advance this happened to me on my 91 many years ago. It lost all its power just dumping fuel in.. it's to the right side behind the tbi and has a little ufo shaped piece on it.. also look up symptoms for vacuum advance not working sounds like the issue. But I'm not sure worth a look