Rough idle on some cold starts
- Thread starter Anthrax
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MaineRunna
Member
A leaking injector would cause an extended crank due to the fuel pressure bleeding down overnight. When the engine starts it would run rough and smoke for a minute until the flooded cylinder clears out. It would probably do this every cold start though. The IAC probably wouldn't cause a rough idle, it would just stall or idle at the wrong speed, maybe if it was stuck and idle was way to low it might run rough until you gave it some gas. Doubt it's the pressure regulator. Have you checked the spark plugs, wires and coils?
19963.4lsr5
New member
My last ruff idle on cold start with lots of white vapor out of the tail pipe was from antifreeze.
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Sent from my iPhone using Tapatalk
Sparkplugs and wires are under a year old. I have not messed with coils. Is there a way to test or clean the coils?
I'm pretty sure its not a leaking headgasket. I'm not losing any coolant and have no symptoms besides the rough idle on cold start that would point to a bad headgasket
No it doesnt look like it. I checked color of all my fluids, have been checking coolant level daily and I even smelled my exhaust and not getting that typical coolant sweet smell out of it
AntleredRuin
New member
Sounds like it could be a fuel issue. Maybe the fuel pump is having trouble keeping up?
I had the opposite issue a few months ago. Fuel pump was failing and it would idle extremely rough on warm starts (sitting less than an hour). If I gave it gas or let it idle for about 30 sec - 1min, it'd go away and idle normally.
This wasn't every time it started, but most times.
Something about a faulty check valve in the fuel pump that's supposed to keep fuel pressure primed even when the truck is off. This one pretty much failed closed and starved the truck of fuel until it finally popped back open again.
I had the opposite issue a few months ago. Fuel pump was failing and it would idle extremely rough on warm starts (sitting less than an hour). If I gave it gas or let it idle for about 30 sec - 1min, it'd go away and idle normally.
This wasn't every time it started, but most times.
Something about a faulty check valve in the fuel pump that's supposed to keep fuel pressure primed even when the truck is off. This one pretty much failed closed and starved the truck of fuel until it finally popped back open again.
Mudeprived
New member
Test the fuel pressure after it sits overnight.
I had the exact same issue and it was a pinhole leak in the headgasket that gradually enlarged over time to the point where coolant loss became obvious and white puffy exhaust smoke appeared.
I had the exact same issue and it was a pinhole leak in the headgasket that gradually enlarged over time to the point where coolant loss became obvious and white puffy exhaust smoke appeared.
Any good guides on testing fusk pressure? From what I remember checking it on these trucks are kind of a pain right? Would a fuel pressure problem automatically point to bad HG?
I have a tiny leak out of backside of valve covers. Not to where I have to top off between oil changes or anything tho
Mudeprived
New member
No, the fuel test is to see if the system is holding fuel pressure.
And yes you need to add a fitting to test fuel pressure. There are a few write ups.
AntleredRuin
New member
There are a few write-ups, but unfortunately they are less than helpful, unless you really want to mod (read: get a lot of pieces from a hardware store and then solder/weld them together), and even then they aren't terribly descriptive.
I've never seen anything that was simply a bolt in tester. The 2 most obvious places I'd try to tap inline into are the front driver's side fuel rail, and the high pressure line on the driver's side of the engine.
You'll need to get some kind of adapter though; all the fuel pressure test kits I rented (or bought and then returned) couldn't tap straight in; the size and the thread pitch was all wrong. I wound up just making an educated guess and replaced my fuel pump (and that fixed my problem)
There's supposedly an FSM instruction to use the front of the fuel rail, with a supposedly 'proper' banjo fitting, but I've yet to seen proof of this, or the part number. It's just hearsay.
thegipper
New member
You'll need a banjo adapter for checking the fuel pressure. I believe pretty much all Toyota's require this (I think Honda also). A fuel pressure test will only really tell you if the fuel pump is weak or not or if the fuel filter is plugged up. If there was a leaking injector (or a line), it would have to be pretty bad to show up on a fuel pressure test.
Let's not jump to conclusions but not all HG failures are the same. Mine was very minor but I would slowly loose coolant over the course of a week (maybe a 1/4" - 1/2" in the reservoir). Mine also didn't smoke on startup but I did have the rough idle (No CEL either). Moral of the story, HG failures are not always obvious but if you aren't loosing any coolant, your HG is probably fine. A "rough idle on cold startup" is a symptom of a failing HG but it can also be a leaking injector and a few other rare things.
This is the adapter you would need.
It allows you to hook up pretty much any standard fuel pressure gauge to the fuel rail on Toyota's/Honda's and I'm sure some other companies use this setup also.
Let's not jump to conclusions but not all HG failures are the same. Mine was very minor but I would slowly loose coolant over the course of a week (maybe a 1/4" - 1/2" in the reservoir). Mine also didn't smoke on startup but I did have the rough idle (No CEL either). Moral of the story, HG failures are not always obvious but if you aren't loosing any coolant, your HG is probably fine. A "rough idle on cold startup" is a symptom of a failing HG but it can also be a leaking injector and a few other rare things.
This is the adapter you would need.
It allows you to hook up pretty much any standard fuel pressure gauge to the fuel rail on Toyota's/Honda's and I'm sure some other companies use this setup also.
Last edited:
96 ToyRunner
Member
Same thing is happening to me but I have a brand new motor and everything is new I’ll post as soon as what is found out what’s going on
Did u have the rough idle during every cold start? Mine seems pretty random to whether it does it or not.
I will monitor coolant very closely and look into getting that adapter.
