Skip the auto shop - Our mechanics make house calls

Q: The truck intermittently, has a very rough idle, and runs rough,usually after warming up.

asked by on

The truck was running fine until a fill up a few weeks ago. Shortly after I filled the tank, it started running rough and idling rough. I ran some sea foam, and then fuel injector cleaner through it. Then changed the fuel filter, and now have changed the spark plugs. I had changed the plugs previously and wires at 100k. The truck seems to run fine when cold, but once at operating temperature, the idle becomes very rough, and sluggish on acceleration. I have also cleaned the M.A.F., the EGR valve, and checked the pcv valve. Motor appears to change and run better if I disconnect the MAP, but not convinced that's the issue. The fuel economy has been about 15mpg for some time, down from 19 mpg when new. Had a mechanic put on a test, and the reading was first a bad O2 reading on cylinder 4. Then, cylinders 1,3, and 5. Sometimes, the Check Engine light blinks. It did cut off (the light) at first when I changed the plugs. O2 sensors have never been changed.

My car has 172000 miles.
My car has an automatic transmission.

A: Hello. This can be caused by a number of di...

Hello. This can be caused by a number of different things. If this occurred right after you had gotten fuel then I would start by testing the fuel pressure. I would do this when the vehicle starts running poorly. It is possible that the bad fuel contaminated the fuel pump and it is not putting out the right pressure. If the fuel pressure is fine then I would check the other sensors readings with a scan tool. One other common thing that I have seen on this engine is a leaking intake manifold gasket. It starts leaking on the bottom side and that leak causes the MAF and MAP to not read correctly which causes it to run like this. If you would like to have this rough running condition looked at, consider having a YourMechanic certified techniciancan come to your home or office to diagnose why it is running rough.

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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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: I have a 99 Yukon when I put it in reverse it goes forward. It has only done it twice

Hello and thank you for contacting YourMechanic. Make sure the linkage on the side of trans moves along with the shifter cable as the cable moves. If that's okay, then drop pan and check the linkage to manual valve...

Q: Turn signal, interior lights, headlights, brake lights, all malfunctioning. new bulbs, new switches

Thanks for writing in about your 2000 GMC Sierra 1500. Symptoms like these can be very confusing. It can almost seem like there is a ghost in the system. As simple as it may sound, you most likely have a...

Q: Trunk not opening even though I hear a click in glove box when pushing the button.

Hello, when you push the trunk release button the clicking sound you are hearing is the relay behind the glove box that completes the circuit to the solenoid that releases the trunk latch. The next step would be to see...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...