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2000 forester new motor, rough running

Hi all, i’m new here i have a 2000 forester that my son and i rebuilt together …the motor(ej25) had all work done by a mech, shop put it all back together …install it… she ran good but with a little roughness at idle… on our first drive out i noticed that the idle was rougher when in drive or park idle it was less??? i have checked for codes,no codes recheck our timing belt marks… timing line up good new fuel filter… new spark plugs and wire …check IAC …check for vac leaks cant seem to find any …check TPS its good… replaced both front axles… check motor mounts there good help thanks Sean

I had same trouble. Any benefit response? :tractor:

Many cars with automatic transmissions idle a bit rougher and slower in drive, but it should smooth out in neutral and park. Is your idle quite slow? Maybe tweak it up just a tad.

hello Henoxifu, not yet… after i posted i thought of more info on the engines history like the guy we bought this car from …had run the engine out of oil as well as it over heated… after i wrote the post i think that i might have a issues with the injectors do to over heating ? i dont know if i am on the right track… i hope someone will help with some info

hello wentwest, Subaru has no idle to reset unless you have the equipment for such a task as resetting the computer. i did do a trains clean, new fluid and filter,… but it did not change …thanks Sean

I find your post a little confusing. You say you and your son rebuilt a 2000 Forrester, but then you say you had all motor work done by a local mech. Mechanic ?, machine shop? who put it all back together and installed it Rebuilt means different things to different people and we can;t guess what was done to this engine that was run out of oil and overheated.

If a local mechanic rebuilt your engine and installed it why aren’t you asking him about the rough idle?


Thanks for the reply oldtimer_11… My son and i put pulled the motor apart and put it back together… the engine had to be bored out 20 over so i had the machine shop do the work. i did the heads (lapped the valves) …reinstalled the motor-
it started up fine… i did ask some local mechanics their thoughts and you can see in my first post what we have done.
today we switched out a manifold from my other forester but got the same results when in drive or reverse. This switching out eliminates the injectors, coil pack, TPS, IAC, spark plugs and wires…so then hooked up a vac gauge which read at 20 but when we put into drive it dropped to18 …oh the other car runs good… thanks again, Sean

Were the valves cut to narrow the seat before lapping, were the valve stems checked for lateral play and the guides for wear, were the springs checked for installed height and pressure, were the cam surface and lift checked?

That is true for I-4 engines, but it is not true for H-4 engines–such as the one in this vehicle.
Subaru’s H-4 and H-6 engines have an extremely smooth idle, unless there is something mechanically wrong with the engine.

the machine shop resurfaced the heads, but i don’t think that all of that was done,… thanks oldtimer-11
i think im going to pull the heads and have then check out… what bothers me is the 180 compression test all around and leak down test where good??? any thoughts… thanks… Sean

If you are measuring intake manifold vacuum in the 18 to 20 inch hg range, that’s a good sign of a healthy engine. Suggest to do a compression test too. I’m guessing you either have a mixture that is a little too lean, or there’s some asymmetry in the engine performance, like one cylinder isn’t doing its job as well as the others. An ignition system, fuel pressure, and fuel injector system check would be the place to start. A tailpipe emissions test might be diagnostic too.

Did you get CV shafts replaced with aftermarket ones by any chance?

If yes, it might be the root cause for a very nasty vibration in D / R / P transmissions selector positions, but idle will be silk-smooth in N.

I used to have 2003 Outback and replaced CV shaft with aftermarket one and had this problem until found a simple solution in Subaru forum: get original one repacked with grease and new boots. Vibration disappeared right after I removed aftermarket shaft and installed OEM one in place.

If all the clearences were held to the tight side, the engine might smooths out after it breaks in. Like cars of the 40s and early 50s.

I see you mentioned 180 compression and a good cylinder leakdown test as well as an 18-20 vacuum reading. Are there periodic brief drops in the vacuum reading? If not, I don’t think you have an actual valve or other mechanical issue and would not pull the heads just yet. Has the machine shop heard the engine and do they have any suggestions.

I am puzzles by the suggestion of bad CV joints, how can a CV joint cause a vibration at idle, no matter what gear it is in?

It was explained quite well on Subuaru forum I’ve read it from and it boiled down to the specifics of Subaru opposite engine/transmission design:

  • if “regular” FWD has crankshaft rotating essentially on the same axis as CV shafts, Subaru’s opposed design makes crankshaft and CV axises to intersect at right angle
  • in any “loaded” transmission selector position, FWD design allows for idle vibration to push against CV shaft freeplay loading/unloading it, Subaru design gets them loaded all the time and pushes against them and through them against body
  • OEM CV shafts are specifically made to have HUGE freeplay and very low friction to allow for normal idle vibration to “stay in freeplay”, rather than get transferred to the body, compensating for this design specifics
  • aftermarkets CV shafts I bought were immediately apprent to have might tighter freeplay and more friction
  • at any “non-detached” transmission selector positions (D/R/P), CV shafts are “pre-loaded” and thus increased (to compare to OEM) friction makes for idle vibration to transfer to body, since clearances are so tight

I was REALLY surprised myself to get that explanation… further, at this point I’ve discarded an original OEM shaft with torn boot, so I had to buy a used one from eBay as I was desperate to get rid of that vibration and already re-checked everything possible but that CV shaft. Replaced it and got it back to “like new” state.

hello i to read that to, but i have a friend with a forester who replaced both cv shafts (after market) and it doesn’t vibrate like ours?? and i wonder if it’s remans that they are talking about, we did replace the cv with new one ( howbe it not Subaru cv’s) with no change… thanks andriy…Sean

OLDTIMER- im not comfortable driving it around yet, so no i have not take it to the machine shop .
for they an hr from where i live thanks …Sean

Some folks have found this kind of trouble be due to dirty engine ground connections. It seems unlikely in your case but you might have a look at that.

ya… i ve been looking as i move around the engine area… good thought…Thanks Cougar,

andri formenko, Thanks for the great explanation, one of the things I love about this site is all the things I learn.