High rev on cold start.. CONTINUED!!!

[hgeranium]

Alright guys, I made a thread previously about this issue, thought snugging up the manifold bolts fixed the problem, but it has returned in full force. I am going to go into much more detail because, at this point, I am at a total loss for what could be causing this high rev issue.

 

I apologize for the length, I figured the more detail the better.

 

The issue plays out like this: I start the truck, the revs shoot up immediately to 2000-2300 and stay there. Putting the truck in a gear drops the idle (like usual) to a cool 1500-1700RPM when completely still. The truck always wants to lurch forward when it is in this state, it is especially scary when in traffic because the revs are fighting the brakes and they're barely enough to keep the truck stopped. It feels like I have my feet on the gas and the brake at the same time. When I turn the truck off in this state, it shuts off hard and shakes the entire thing having to abruptly stop the motor, which can't be healthy for it.

 

For this reason, one of my suspicions is that the throttle plate is getting partially stuck open. This makes sense logically as to explain my symptoms but I am more at a loss of what could be causing it.

 

The most difficult part is it's intermittent. I find that, rather than it differing from startup to startup, the problem can  persist for a full day or two and go back to normal for however long. Turning it off and back on does not fix the issue in most cases. I have had instances where the increased idle occurs for 10 seconds, 30 seconds, 1 minute, 5 minutes, and the entire length of every trip I've taken throughout a day (20-30 minutes driving total). The added inconsistency to the already intermittent nature is making it very frustrating. In the cases where the problem resolves itself, I see the revs crawl back down to a normal state and it drives perfectly fine afterwards.

 

Even in cases where the revs don't get up to the 2000s, I've seen my idle in park hang out around 1200-1400 and my idle in a gear is right at 1000. This is much easier to deal with when trying to drive because I'm not fighting the truck to stop/slow down nearly as much, but I know it is the same issue occurring at a lower intensity.

 

This also makes me think it could be some vacuum issue in the manifold gasket itself? If there's not a full seal, it makes sense that extra air would get sucked in and raise the idle, with the problem fixing itself in the rare cases that the leak gets a good enough seal to vacuum it off. All my other vacuum lines are good (tested with brake cleaner, no change in idle), but I haven't gotten around to checking the manifold gasket with brake cleaner/water since I'm writing this not long after it happened. It's on my to-do list when I have a morning off.

 

Relating to this ^ are there any tips/how to for removing the manifold heat shield? I don't want to have to mess with the injectors/fuel rail to get it off, but I don't see another way nor do I see any guides online. 

 

I'm going to list related or adjacent fixes or checks that I have done pertaining to the idle so y'all know what has been done and not worked:

New TPS (voltage tested), new IACV, cleaned throttle body (not taken off and fully deep cleaned; I have the gasket and is on my to-do list) and sea foam treatment, manifold bolts all snugged down (top of gasket vacuum tested w/ BC but not the bottom yet).

 

It may not be related but all my sensor grounds are good, no resistance anywhere (cruiser tip 5).

 

I remember since I've owned the truck from 2022, this problem was a once-a-month (if that) occurrence, but in the past few months, seemingly out of nowhere, the issue has become exponentially more common and severe (multiple days a week, multiple times a day). This few week period between my original post and this post is the longest I've gone without the throttle sticking once in a good few months.

 

I have a satellite "issue" where, even if it idles normal on startup, it'll still rev up to the 2000s and come right back down. I am willing to bet that the cause for the high idle is also causing this issue. The amount it goes up is inconsistent and sounds different every time, so it seems more of an issue rather than a quirk. When neither of these issues occur, then at cold start, the RPMs will go up a normal amount and come down fast enough to kill the motor if I don't blip the throttle. This happens in all seasons, doesn't matter if it's cold outside, just the engine being cold.

 

That's all I have right now. I am going to start recording every startup to capture some evidence and add onto this thread with those along with any attempts at repairs. I will be happy to share any more details if needed.

 

TL;DR: Throttle stuck open on startup intermittently, I'm thinking either there is an intermittent vacuum leak in the manifold gasket or the throttle plate itself is getting stuck open.

[Eagle_SX4]

High idle is almost always caused by a vacuum leak. 

 

Your IAC valve may be getting stuck open and only occasionally working. New parts are not always good parts.

 

It is easy to check you if your throttle is getting stuck just remove you intake tube that is bolted on to the throttle body (3 torx bolts) and look inside. You can start the engine with that removed. With the engine off cycle the throttle by hand and see if it likes to stick at any point.

 

If you suspect a vacuum leak pull off all of the vacuum hoses on the intake and plug the holes. If you idle returns to normal then add them back one at a time until you find the issue.