Rhythmic engine surge 4.8is
 

Hello,

My 2005 4.8is, 120K miles, engine runs smooth, but at cruise there is the slightest engine surging, rhythmically about every two seconds, barely even noticeable, but it is there.

At idle there is a very noticeable change in idle speed, about 100 rpms, it does not occur as consistently as at cruise. Maybe every five seconds or so.

Engine has new valve seals, Upper timing cover gaskets, valve cover gaskets, air pump rebuild, Ccv and hoses, alternator bracket gasket, oil cooler line o rings, and all the associated items that would go with these repairs. I was hoping my issue was cause by vacuum leaks from one of the gaskets, they did not fix my problem.

I also changed the minimum valve lift, (I tried this after searching this forum as a possible fix) The valve lift was set at 1mm, I tried 3mm, and also 8mm. It did not fixed or change my problem.

No engine fault codes from INPA.

Anyone have any ideas?

Could it be a Mass airflow problem? Any other common vacuum leaks? Vacuum Lines look good. Help please!?

Bump

Good day!!

It could be as simple as the A/C compressor kicking on and off... See if it still happens with the climate controls off...

Cheers!!

you might find your answer here:
The dreaded engine surge - Automotive Service Professional

And here:
https://www.2carpros.com/articles/engine-surges

Thanks for the thought, but that definitely is not it

Thanks. Obviously I knew it could be a lot of different problems but I was hoping there was a common 4.8is problem! I appreciate the comment. Time to start looking!

My 4.4 does something very similar. I was thinking along the lines of a coil intermittently misfiring when it gets hot. I haven't gotten around to diagnosing it, but that's where I'm going to start. It feels electrical to me. Or possibly an injector intermittently not opening enough.

My 01 4.4 does the same thing. I notice it mostly when getting on to a highway. Partial but intentional throttle application. Sometimes it isn't there at all and is when I realize how much I love that engine! I have changed fuel filter, live streamed data looking for something, monitored fuel trims. I can't nail it down. Was thinking of sending injectors out to be cleaned. My motor is fresh with all new vanos, guides, CCV, gaskets. I have smoked it and nothing. This seems to be showing up on posts more and more. Wish there was a way to find a common denominator for all of us, other than these cars are starting to get up there in years.