Key word is "supposed". I also believe the penetrant can get past the gasket. That is why I said that I do it over time. I know it is not instantaneous.
Engine Idles Unevenly and Almost Stalls When Cold
- Thread starter cc333
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Always good to have a known good O2 sensor.
if you want some confidence on whether the O2 is a contributor check whether problem only occurs in open loop goes away in closed loop. If problem is occurring in open loop only it’s not likely caused by O2 sensor.
The O2 is only used for afm after getting into closed loop.
Of course, you could have a bad O2 and never being able to get into closed loop. Old engines often have more than one problem which are not noticed until chasing a more obvious problem. I just mean that a bad O2 is not going to cause a idle hunting up and down if never getting into closed loop.
Adding to the “it’s worth investing in data logging capability”. You can check if the O2 is working by watching the output as the PCM changes the afm target. The PCM moves the afm up and down and expects to see a response in the O2. If it doesn’t reach expected values, too slow to respond, & some other stuff it will drop out of closed loop. I had a “lazy“ O2 sensor once which was dropping me out of closed loop during hot idle and causing me to failing emissions. It would get into closed loop on cold start fine, stay in closed loop driving but drop out on hot idle which is part of the emission test.
That said, getting going on data logging takes some time and effort. Worth it if you are planning to keep the truck for a long time. Probably too much time when in middle of problem now but something to think about for the future.
for now, I’d narrow done whether the problem is in open or closed loop or both before messing with O2. They aren’t that expensive but sometimes hard to remove. Easier to use a paper clip to decide if it’s a candidate first.
Well, I ran the truck, locked into closed loop, until it warmed up a bit, and it was idling like normal, unlike before, when it goes into open loop.
So, this suggests an iffy O2 and/or possibly ECT sensor, doesn't it?
If needed, at the next opportunity, I can blink out the codes next, after I cold start it like normal without the jumper.
c
So, this suggests an iffy O2 and/or possibly ECT sensor, doesn't it?
If needed, at the next opportunity, I can blink out the codes next, after I cold start it like normal without the jumper.
c
My O2 sensors weren’t giving me any issues, but I pulled all 4 last summer and replaced all. Increased hwy mpg by 3-4. Mostly, I just wanted to pull them before they were fused to exhaust and glad I did. The originals only had about 80k on them, so I soaked them in gas, cleaned them up a bit and careful not to poke anything in the actual “sensor” area. The sealing gaskets however need to be replaced, I think I’ve found a source for them.
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grampadirt
On MEDICARE Officially Old
I believe that's normal because mine does the same thing and it's done it for fifteen years.What's happening here is when the engine reaches a predetermined temp the Idle Control Solenoid releases the diaphram on the side of the injector pod and the idle drops suddenly causing the engine to almost die.Do this....remove the air cleaner,start the engin(when cold)and watch that diaphragm and see what it does.Do this when engine is cold and not moving.Might take a few minutes.1994 GMC Sierra K2500 w/ 7.4L.
It had been serviced thoroughly a couple years ago with new distributor, new plugs and wires, and a bunch of other things I don't remember anymore.
It runs well once it's warmed up, but on a cold start, it will idle high for a bit like normal, and then when it drops down to low idle kind of suddenly (is that normal?), it starts fluctuating and getting close to stalling, and then it catches itself and revs up again (presumably this is the IAC doing its job).
It does this for a few minutes, and then it stabilizes once it gets up to normal operating temps, at which point it's fine.
It seems to do this in any weather, but it's more pronounced when it's cold (which, in California, isn't saying much).
The ECU died back in early 2020, at which time I bought a replacement (I transferred the CALPAK, which seems to work at least as well as it did before the ECU conked out, so I assume it's still good?). Could this have anything to do with it? I seem to remember that that was about when the weird idling on cold starts began, but I'm not totally sure. Could the replacement ECU be faulty somehow?
A possible clue to that effect is that the SES indicator will sometimes light up briefly during the rough idling episodes (it will go out when the idle evens out). I don't know the code.
Thanks!
c
The same two pins in the diagnostic connector that you'd connect to get the blinky-light codes?
That doesn't put the truck into open loop. It does tell you if the mixture is rich or lean, and whether it's in open or closed loop.
If the light is "on" more than it is "off", the mixture is rich. If the light is "off" more than it's "on", the mixture is lean.
If the light flashes "fast", about two and a half times per second, the truck is in open-loop. If the light flashes "slow", about once per second, it's in closed-loop.
The V-6 and small-block engines should be in closed-loop at idle once the O2 sensor has warmed-up and the open-loop timer has run out--a couple minutes after startup. Often they are not in closed loop--faulty/lazy O2 sensor is typical cause. In 1988, the 454 will be in open loop at idle, and that's normal and OK. I don't know if that changed by '94.
You didn't lock it into closed-loop. Can't be done.
If you disconnected the O2 sensor, you can "lock" it into open loop, but that'll set a code.
Are you mixing-up open vs. closed loop? The truck should start in open loop, and switch to closed loop when the O2 sensor is heated up enough to function. The vehicle spends most of it's time in closed-loop in normal operation.
It "could" be about a dozen things. Yeah, you can get the blinky-light codes. Codes can be helpful. The real power is in the diagnostic "data stream" which requires a scan tool instead of a code-reader or paper-clip.
Nice! And not that uncommon.
Wisdom.
Ehhh. Not so sure about "cleaning" them, particularly in gasoline.
Sealing washers are as close as an 18mm spark-plug gasket.
www.aircraftspruce.com/catalog/eppages/spgaskets2.php
Copper Spark Plug Gaskets - 18MM | Aircraft Spruce
Copper Spark Plug Gaskets - 18MM AN4027-1, 18MM P/N M674. These are the AN4027-1 18mm copper spark plug gaskets. Made of solid copper to current aircraft standards.
Realistically, I'd just re-use 'em. Glop some anti-seize on the threads and screw 'em back in...IF (big IF) you think they're still usable. I figure if the mileage went up that much when they were replaced, I'd put the old ones in the scrap-metal bin.
Nope.
There is no "diaphragm" for idle speed control, and it's not supposed to be on/off like you describe. Idle speed should not drop "suddenly". And the moving parts aren't visible, either.
The "diaphragm" on the side of the injector pod is the fuel pressure regulator. The nearest diaphragm besides the regulator is the EGR valve. You're looking for the cylindrical IAC motor on the side of the throttle body.
The Idle Air Control motor will vary the size of the idle air passage, from fast idle to curb idle, smoothly. Your IAC motor is sticking or faulty, or the wire harness to it is faulty.
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I’m not sure either, it did clean them up some. There’s a bunch of online information showing this, but who knows. I saved them, because like I said, I wasn’t having issues with them and who knows, I just might pick up another 400 and these might be an upgrade for cheap. The sealing washers I found are 18mm.
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grampadirt
On MEDICARE Officially Old
Then explain these....and the fuel pressure regulator is on the rear of the injector assembly.The same two pins in the diagnostic connector that you'd connect to get the blinky-light codes?
That doesn't put the truck into open loop. It does tell you if the mixture is rich or lean, and whether it's in open or closed loop.
If the light is "on" more than it is "off", the mixture is rich. If the light is "off" more than it's "on", the mixture is lean.
If the light flashes "fast", about two and a half times per second, the truck is in open-loop. If the light flashes "slow", about once per second, it's in closed-loop.
The V-6 and small-block engines should be in closed-loop at idle once the O2 sensor has warmed-up and the open-loop timer has run out--a couple minutes after startup. Often they are not in closed loop--faulty/lazy O2 sensor is typical cause. In 1988, the 454 will be in open loop at idle, and that's normal and OK. I don't know if that changed by '94.
You didn't lock it into closed-loop. Can't be done.
If you disconnected the O2 sensor, you can "lock" it into open loop, but that'll set a code.
Are you mixing-up open vs. closed loop? The truck should start in open loop, and switch to closed loop when the O2 sensor is heated up enough to function. The vehicle spends most of it's time in closed-loop in normal operation.
It "could" be about a dozen things. Yeah, you can get the blinky-light codes. Codes can be helpful. The real power is in the diagnostic "data stream" which requires a scan tool instead of a code-reader or paper-clip.
Nice! And not that uncommon.
Wisdom.
Ehhh. Not so sure about "cleaning" them, particularly in gasoline.
Sealing washers are as close as an 18mm spark-plug gasket.
www.aircraftspruce.com/catalog/eppages/spgaskets2.php
Copper Spark Plug Gaskets - 18MM | Aircraft Spruce
Copper Spark Plug Gaskets - 18MM AN4027-1, 18MM P/N M674. These are the AN4027-1 18mm copper spark plug gaskets. Made of solid copper to current aircraft standards.www.aircraftspruce.com
Realistically, I'd just re-use 'em. Glop some anti-seize on the threads and screw 'em back in...IF (big IF) you think they're still usable. I figure if the mileage went up that much when they were replaced, I'd put the old ones in the scrap-metal bin.
Nope.
There is no "diaphragm" for idle speed control, and it's not supposed to be on/off like you describe. Idle speed should not drop "suddenly". And the moving parts aren't visible, either.
The "diaphragm" on the side of the injector pod is the fuel pressure regulator. The nearest diaphragm besides the regulator is the EGR valve. You're looking for the cylindrical IAC motor on the side of the throttle body.
The Idle Air Control motor will vary the size of the idle air passage, from fast idle to curb idle, smoothly. Your IAC motor is sticking or faulty, or the wire harness to it is faulty.
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Ah, I see.
OK.
Apparently so.
OK.
How can I accomplish that without hunting down expensive and hard to find dedicated equipment?
As far as a PC-based solution goes, I have a variety of laptops with serial, parallel and USB that can run anything from Windows 9x to 10, so I can use whatever software and hardware, new or old, is available, so long as it is relatively cheap.
c
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