so I started up the truck and was working on bleeding the system on a hill, and my truck is hiccuping every few seconds, the rpms are dropping suddenly and regularly.

While searching I learned that this might be an air bubble hitting the the coolant temp sensor, but it's strange that it's on some regular interval. Anybody have any thoughts?

 

Mine is doing the same on cold mornings then stops when completely warmed up. I am suspecting a air pocket in the coolant.

 

yeah it's still doing it intermittantly, stops when I rev the engine, but then starts up again. I also noticed that when I let off the throttle at speed I'm getting a sudden drop in RPM's, feels kind of like a shift in the gears....strange. I though I'd bled the system adequately.

 

If it was an air bubble (Which I highly doubt) It would run faster when the sensor thought the engine was cold because the ecm would increase idle speed and fuel, or so it seems.

 

When the ECM is getting a cold signal, it will send more fuel. My theory is that temp sensor is getting a false cold reading. A rich A-F ratio in warm engine will decrease RPMs. When my engine is completely warm it will idle correctly, possibly because the air pocket is compressed and coolant is reaching the sensor. If the temp is above 50F my engine does not lope at all while warming up. My engine didn't behave this way until I change the radiator last year.

 

That's one amazing air bubble, It hits the temp sensor at regular intervals and stays in the same spot all the time. If you really believe that's the problem, why not take out the temp sensor let the bubble out and put it back in and that should do the trick, that is if it's an air bubble problem.

 

That is exactly what I plan to do when I have enough time to remove parts to get access to it. I have read on different forums where people have the same symptoms caused by the air pocket. The loping is caused by sensors working against each other, giving the ECM conflicting information not because of a magical air bubble. lol, I thought the same as you when I read about it the first time, but now I am not as skeptical.

 

yeah I think it's strange behavior for an air bubble as well. When I had the radiator cap off trying to bleed it again, I noticed a pulse in the level of the coolant going with the change in the rpm's, which I guess is to be expected.

my issue is definitely sticking around though, when the engine is hot, cool, whatever.

 

oh here we go:
"Problem solved and answers found! After installing a new water pump, I paid really close attention when filling/bleeding the system. With the truck on ramps and the engine running, once the thermostat actually opened, the engine started surging again and I noticed the level in the radiator pulsing with the surges. The first time this happened I had the radiator cap on, the second time it was off and I was watching it. Once it began to surge the radiator gushed out coolant in surges. I tried squeezing the hoses to help burp the air out but it kept surging. Finally I got frustrated with the splashing coolant mess and shut it off. Only then did a really big burp let out and the level of the radiator dropped a lot. After that, problem solved; topped off the radiator, no more surging, cool running, all good."
https://www.yotatech.com/forums/f116...p-fail-194565/

I've definitely got some air gurgling in my heater core too.

Can I get this out just by parking on a hill or do I need to start messing with hoses?

 

You should have had the heater on when you burped it.

 

TNRabbit is right, heater on!

 

the heater's been wide open since the flush.

 

new development:

@#$%! I'm leaking coolant! that's probably how the air is getting in the system!

it's dripping off the differential...can't see the leak. I hope I didn't blow out the water pump seal when I flushed it...

btw, something I discovered, if I switch to 4wd the idle issue goes away...apparently the idle system works differently in 4wd?

 

Ive been having this idle issue for a few years, only when its cold. And loosing water, lots recently. I flushed the system when a plastic piece decayed and sent chunks into my heater core. Hope mine gets fixed when I find this damn leak.