Rough idle/misfire when cold until warm - Re-Solved - HG

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Sound like a vacuum leak. That seals once warmed up More times than not it a head gasket starting to go. Doing a warmed engine compression test will not show it . Do a cold compression test and a leak down I see an average of around 120 cold . Could also be a seal leak on an injector or intake that seals once everything is warmed up. I have spent endless hours chasing these problems and you have to chase them when cold. I have found a propane torch (unlit) with a hose on it is great for finding these types of leaks . You can get the hose all under the intake with pretty good accuracy to know where the gas is going if the engine changes ..
 
raisinhell2 said:
Sound like a vacuum leak. That seals once warmed up More times than not it a head gasket starting to go. Doing a warmed engine compression test will not show it . Do a cold compression test and a leak down I see an average of around 120 cold . Could also be a seal leak on an injector or intake that seals once everything is warmed up. I have spent endless hours chasing these problems and you have to chase them when cold. I have found a propane torch (unlit) with a hose on it is great for finding these types of leaks . You can get the hose all under the intake with pretty good accuracy to know where the gas is going if the engine changes ..
Click to expand...
I did a cold compression test. I did spay down some brake clean around TB to see if anything would change but no change.

Unfortunately it doesn’t just do it when cold anymore. It did it to me after I drove for 15min and parked it for 20.
 
My failing head gasket started with rough idle at startup. I didn't know what was happening at first. Rough idle would go away after warm. The second symptom was engine running slightly warm followed by small but gradual loss of coolant. Eventually, my engine had coolant pouring into cylinder 6 and almost became hydrolocked. I was able to get it started and make it home but it was running super hot.
 
My first guess was also vacuum. Part of the vacuum switching under the intake manifold, EGR temp sensor, and the bimetal valve in the block, limit EGR and vapor canaster intake until the engine warms up, in order to avoid running rough. A leak or a bad vacuum switch could cause what you describe.

The FSM has solid vacuum trouble shooting guidelines.
 
That still makes me think of vacuum lines or throttle body gasket. But, it sounds like you already troubleshooted that.

If your IAC is on its way out, it can:
  • Stick when cold
  • Then stick randomly when warm
Symptoms:
  • Idle hunts or drops
  • Sometimes stalls at stops
  • ECU can’t stabilize idle anymore
 
PeterTheBeater said:
This very cold morning had the same rough start, this time it threw a cyl6 misfire, going to shove a boroscope into it when it’s cold.
Click to expand...
That's a good idea. Was it driven much the day before? If it is a HG issue then it will be likely exacerbated by getting the engine properly up to temp before it cools.
 
LandCruiser105 said:
That's a good idea. Was it driven much the day before? If it is a HG issue then it will be likely exacerbated by getting the engine properly up to temp before it cools.
Click to expand...

robnicko said:
if possible before starting it, remove and have a close look at the spark plug as well for any coolant residue
Click to expand...

The spark plugs all looked the same and compression was OK
 
Have you checked your TPS rigging as per the manual?

I had a similar, not exactly the same, experience and it was the TPS, as odd as that sounds!
 
It’s the head gasket, cyl 6
Image_2026-01-07 11_37_45_225.webp

263k

Owner since 163k, only Toyota coolant, blue fan clutch, aisin water pump, aluminum radiator, coolant filter, never overheated, ScanGauge for coolant. Serviced frequently and with best parts.
 
Last edited:
PeterTheBeater said:
It’s the head gasket, cyl 6
View attachment 4063264
263k

Owner since 163k, only Toyota coolant, blue fan clutch, aisin water pump, aluminum radiator, coolant filter, never overheated, ScanGauge for coolant. Serviced frequently and with best parts.
Click to expand...
That's about the same sort of mileage mine failed at... With a similar background of ownership and decent maintenance too. I'm sorry to learn it turned out to be HG. On the plus side, you might have caught it early enough not to have overheated and warped the head.
 
Shame but it happens.
Pls do not be tempted to use any of those pour in sealers.
Given you mentioned that it runs fine once warmed up sounds like you have caught it early. Just take your time, remove the head and have it tested & refreshed and put a new HG in.
Mine went at 480,000kms and I too was lucky as it happened about 2 km from home. Cyl 6 as well and nicely steam cleaned when I took the head off

Its not too hard a job if youre mechanically minded but its a rewarding thing when you get it all back together.
 
chasing pretty much the same symptoms ... just curious , other than idle was it gutless at some point ? PO said the HG was ok on the truck but I really doubt it...White smoke or steam is quite thick on startup :(
 
You must log in or register to reply here.

Share this page

Back
Top Bottom