Skip the auto shop - Our mechanics make house calls

Q: Idle random misfire, very light but not passing smog

asked by on

I had just done a head gasket job and started up great ran it for a bit and got misfire code and variable valve timing code and knock sensor #2 on snap on tool. tried to pass smog and it passed 25mph test but failed idle HC:263 , CO: .02% , Co2: 7.4% , O2: 10.0% , NO ppm: 32

i have smoke tested before those numbers and fixed all leaks but still cant find anything if someone could push me in the right direction because 3 other mechanics have taken my money and left me in the same spot. i have replaced o2 sensor, checked coil ohm readings, new spark plugs, new oil, valve seats are good, compression 180 across the board, pistons looked great with no wear during rebuild, camshaft position sensor, fuel filter, fuel pump pressure at 50psi on 3.5 bar system, only thing i could think of maybe injectors or coolant temp sensor but i have obd1 system so am having a hard time without a snap on scanner please help.... Robert

My car has 200000 miles.
My car has a manual transmission.

A: Hello, thanks for writing in. Random misfir...

Hello, thanks for writing in. Random misfires can be caused by many things, and can sometimes be difficult to diagnose for this reason. If you’ve already checked out the listed components then it may be possible that the coolant temperature sensor or injectors are causing the issue. If the coolant temperature sensor or circuit have an issue, then the signal may be compromised and throw off the engine computer’s calculations. A bad sensor can result in an excessively rich or lean condition, which can cause misfires. It may also be possible that one or more of the injectors may be dirty or clogged, and needs to be cleaned or replaced. This is more likely if they are the original injectors, and if the car ever sat for an extended period of time. I would start by testing the coolant temperature sensor, and then going from there.

If you would like further help, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make or suggest any repairs as needed.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Heater valve stuck.

The heater control valve is mounter under the hood on this vehicle. The valve should be closed when the temperature is set to cold for the rear a/c system. The recommendation would be to have a qualified mechanic check for...

Q: Is it possible to install my broken serpentine belt back my car?

Hi, thanks for writing in. I'm a little confused by your question. If you're asking whether it's possible to install a broken serpentine belt on a vehicle and drive 20 miles, I'd say not. If the belt is physically broken,...

Q: I changed the crankcase vent tube, now the transmission won't shift

If the transmission was shifting fine prior to replacing these parts, then you need to start by double checking your work. Something may have been left loose or may not have been installed correctly. It is possible that the transmission...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.