Hey Guys I Need Help Quick
- Thread starter Nick88
- Start date
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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.
Yes. 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?
Last edited:
Caspian1911
Newbie
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 look
Optional recheck to verify Knock Sensor troubleshooting theory:
After checking for any stored codes in your computer, you could try the following:
1) Reconnect the Knock Sensor.
2) Disconnect the battery for ~30 seconds. (Clearing codes, back to pure factory settings)
3) Reconnect battery.
4) Start Blazer. Note: It may have minor driveability issues until it relearns, disregard.
5) Drive Blazer until it starts misbehaving again. Now disconnect the Knock Sensor
again. If it starts driving right again for awhile (before that ECM KS test runs again)
then I would have enough confidence to proceed with replacing the sensor that 'has
drifted out of tolerance but *not* in a way that the ECM algorithm can detect'.
NOTE: It is way easier to monitor the total timing that the ECM has calculated via
a pro strength scan tool. (And for those reading along, it is way way way easier
to have monitored the total timing that the ECM was generating back when the
vehicle was running perfectly, and either take a photo or put pen to paper and
write down the 'known good' ignition timing numbers for future comparison.
And I know this to be true.
On the other hand, I think that it's still instructional to show how we can
figure out a Closed Loop error by temporarily forcing the engine into Open
Loop mode and noting any difference in behavior before the ECM catches
on and goes into self-protection Limp Home mode.
****
And if the truck doesn't duplicate the good running the 2nd time we try
this test? Then that's all I got, and I would change my approach & get
whatever scan tool the TBI gurus recommend and start working the issue
smarter instead of harder.
But since we can reset the ECM back to factory settings & attempt the
retest for free, why not? Maybe we can get your Blazer running right
while you look for a solid OBD1 scan tool for future use?
Good on you for keeping that K5 Blazer rolling.
Hope this helps. Cheers --
phantom 309
OBS Enthusiast
Don'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,.
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,.
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 over
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... Old
evilunclegrimace
Does not always play well with others
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 problem
Russ B
Newbie
Did not see anyone mention checking timing (idle and fast), use vacuum gage. What is condition inside distributor, plugs, wires. All else fails check compression.
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