Higher revving on warm start

[hgeranium]

I haven't had any issues with my throttle sticking lately after doing all sorts of fixes, but that has allowed me to notice a pattern. On cold starts, the truck will start up (a lot of times in very few cranks) smoothly with a quiet and reserved rev up and back down (~1500RPM). I get this turning the truck on before and after work. When it's warm and I am constantly turning it off and on (running errands), The revs are significantly higher and the startup feels more 'jolty' rather than smooth, and always takes way more cranks to start than when cold (revs closer to 2000-2200). I am going to assume that the new IACV is indeed good and not crap out of the box. Yes, my manifold bolts are tightened. All vac lines are good. I am no longer (as of right now) having an idle issue, it's just with startup. I know my truck and I know it's not normal. It hasn't always started like this, cold or warm.

 

I'm thinking that the heat of the engine is exacerbating an issue regarding the intake/exhaust manifold. Maybe the heat is expanding a small intrusion in the manifold gasket that allows for a small vacuum leak on startup, that is absent on cold starts as the cold gasket compresses and seals the gap. If I'm having a vacuum-related issue that is contingent on engine temperature, I feel like it has to be the manifold gasket, right? At this point, I've suspected it enough that I might as well do it just to cross that off the list. I mostly wanted words of affirmation that I could be on the right track because if I'm about to do this job I want to be confident that it's going to fix stuff lol.

[ghetdjc320]

For manifold gasket, grab a Remflex. They seal up exceptionally well.
 

[hgeranium]

17 minutes ago, ghetdjc320 said:

For manifold gasket, grab a Remflex. They seal up exceptionally well.
 

Thank you! I'll spend all the extra money I can to make sure I never have to do this job again lol

[ghetdjc320]

14 minutes ago, hgeranium said:

Thank you! I'll spend all the extra money I can to make sure I never have to do this job again lol

The remflex don’t cost much more you just have to order them in. 

[89 MJ]

Do you still have the metal heat shield for the fuel rail?

[hgeranium]

Wow, I've made I think 3 separate posts on this same issue. It's come back again so I'll just necro this thread instead of starting another.

 

After putting in my third IACV last November, this issue seemed to have subsided (coincidentally or not). I sort of forgot about it and it was running and starting fine. Sometimes on the first or second crank and very smooth which was unheard of. Anyways, as of a week or two ago, the thing has returned. I finally had enough wisdom to get a video of it: 

This is a mild case because the needle came back down quite fast. In the past it would stick for minutes or entire trips at a time, driving like the throttle is partially open. Brakes while in drive were barely enough to keep it still. The problem hasn't gotten to that point in this cycle just yet but it's getting there. I had the idle stick at 2100-2200 for 15-20 seconds a few days ago. I've noticed that this issue will creep up in severity, get so bad that the truck is nearly undriveable, then go away again for a few weeks or months. Putting it in reverse/drive usually always fixes the idle RPMs too, which is strange (except in the cases where the throttle sticks). This specifically really makes me think it's the crappy IACV failing me intermittently 
 

I'm on my third IACV, with that being said it is a standard ignition unit and three duds in a row is not unheard of. I can get a mopar sensor but it will surely cost me quite a bit of coin, so I don't want to shell out for it if I don't have to.

 

Manifold bolts are all tight, but I still have not replaced the gasket. I have the tip on getting the remflex already. The gasket looks decent from what sticks out from the corners but I know that doesn't mean much for what's actually doing gasket stuff.

 

Throttle position sensor is new as of a few years ago and voltage is set correctly. I haven't checked the voltage recently so I may double check it soon just to make sure.

 

I have a very simplified vacuum system. HO air box with the EGR delete. PCV to intake is fine (new PCV hose and grommet), MAP to TB is sort of sketchy because I have yet to do cruiser's fix on it but I don't think it's leaking at all. Spraying brake clean directly on the hole doesn't mess with the idle at all. FPR to intake is also in perfect shape.

 

I still have the EGR itself on there but I have a brand new gasket and it's all capped off accordingly. When I have some spare coin I'm planning on getting boxyjeep's EGR plate.

 

Brand new TB gasket

 

With all this being said, I'm thinking it either has to be another failed IACV or the manifold gasket. The cyclical nature of this problem is what confuses me, though. Could it be another sensor like the MAP going bad? I wouldn't think so because other than the startup, it drives and idles perfectly (until it doesn't). It really does feel like a vacuum issue or the IACV getting stuck and letting a ton of air in, mimicking a slightly open throttle.

 

At this point I think I may just have to bite the bullet and get a new intake gasket to at least cross that off the list. I just wanted some final opinions before I order the remflex.

 

[hgeranium]

Mini bump. Problem is continuing to get worse so I would appreciate some feedback on if the IACV/man gasket hypothesis sounds OK, or some other cause/sensor 

[Ωhm]

During WARM-UP MODE OPEN LOOP, a faulty Coolant Temp Sensor (CTS) causes the ECU to look at the Manifold Air Temp Sensor (MATS). ECU will add ½°C for every 85 RPM's seen by the ECU. Takes time to figure this out. If you're holding onto a REM, you should know right away.

[hgeranium]

10 minutes ago, Ωhm said:

During WARM-UP MODE OPEN LOOP, a faulty Coolant Temp Sensor (CTS) causes the ECU to look at the Manifold Air Temp Sensor (MATS). ECU will add ½°C for every 85 RPM's seen by the ECU. Takes time to figure this out. If you're holding onto a REM, you should know right away.

No REM on hand but I do have a spare CTS that I have yet to install. The one that supplies the ECU specifically. I could throw it on for good measure since I already have one.
 

I still have the issue even when it’s warmed up. Does that make the CTS any more or less likely?

[Ωhm]

CTS is a problem sensor to work with. Maybe not so, 38 yrs ago. It's a plug for coolant and it's threaded into the block. One mistake and your block's in trouble. If it does come out, make sure to rid the passage of all casting debris.

 

Disconnect CTS connector (C222) and using a DVOM (don't use auto range) measure Ω's across the CTS (C222_A is GND side). Watch meter during cold start, warm up and then idle. Look for opens, shorts or maybe no movement at all.

 

[hgeranium]

3 hours ago, Ωhm said:

CTS is a problem sensor to work with. Maybe not so, 38 yrs ago. It's a plug for coolant and it's threaded into the block. One mistake and your block's in trouble. If it does come out, make sure to rid the passage of all casting debris.

 

Disconnect CTS connector (C222) and using a DVOM (don't use auto range) measure Ω's across the CTS (C222_A is GND side). Watch meter during cold start, warm up and then idle. Look for opens, shorts or maybe no movement at all.

 

This is a very dumb question, but are you recommending I should unplug the sensor and probe the connectors with the engine running all the way through cold start to warm idle? Or are you saying to check the resistances at varying temperatures with the engine off (like check it when it's cold, let it warm up then turn it off and check it etc.)? I just would be worried about running the engine for an extended period of time with the thing unplugged. As you can probably tell, electrical diagnostics are not my strong suit, so what may seem obvious to most people is certainly not for me.

[Ωhm]

No. Good question. Either way should work. Myself I'd disconnect the CTS, connect DVOM on CTS pigtail (not the harness) and watch the Ω's. Keep in mind the ECU will default to using the MATS for CTS values and adjust the temperature accordingly (simulated Engine Warm-Up).

[hgeranium]

I need a few more days of consistent evidence but I believe I have found the culprit of a problem that has been plaguing me since I bought this truck four years ago. Since it was something intermittent and idle related, it mostly made sense for it to be the IACV. I’ve never been able to find any vacuum leaks. I would replace the IACV and the truck would go back to normal for a little bit.
 

however, the IACV pigtail has always been missing its retaining clip. It just slides in and friction holds it together. I never thought it was an issue because since the connector was pointing down, I figured gravity was also holding it in place.
 

After wiggling around that connector, the high rev starts vanished immediately. I have started the truck eight or 10 times since then and it has started perfectly. So, I bought a replacement pigtail that has a retaining clip and hopefully this should be it. I guess it makes sense that the engine vibrations would move the connector and pins around just enough to ‘unplug’ the IACV. 

 

I’m assuming the plug would move out of place after driving, cause issues on startup, then the vibrations would potentially reestablish a connection until the process repeated on the road. I’m also willing to bet that a new IACV seemed to fix the problem temporarily  because I would unplug and re-plug the connector more snugly.

 

Hopefully I’m not coming back in a few days to report that the problem has returned, but I absolutely have no vacuum leaks and the CTS looks OK. Plus, this sort of occurrence is the only thing that would explain the intermittence of this whole problem

 

I should’ve used Occam’s razor to check the simplest solutions first, but this sort of thing never occurred to me. Maybe if I had mentioned that the connector didn’t have a clip, somebody would’ve told me way earlier lol.