Car shakes above 55mph

[SWRT]

Anyone have this happen? It's not a suspension thing.
I believe it's still my engine from the old problem.
I had my alternator/battery connected and accidentally
dropped it on the radiator support and popped the fusible
link. Hasn't run right since. Everything ran perfectly before
that. It's a swapped, clean engine with all new seals and
doesn't leak a bit or make any bad sounds.


Now it has thrown a check engine light twice. Once for
about 5 seconds and another for about 20 seconds.
That's not enough time to get in the dash and connect
the diagnostic cables.


Any speed above 55mph and the car starts shaking
violently. Then it'll do a few light backfires (think of
lighting butane at the bottom of a long tube) and
if you let off it calms down. Then you start to give it
a little bit of pedal and it shakes again. It does this
probably 3 or 4 times, then usually runs smooth
again. It's not a suspension issue because it runs
nice and smooth after the engine has it's episode.
It does run slower all the time though. Feels like it
should have a lot more power, but it's being held back.


So far I've replaced the ECU, the knock sensor,
cam sensor and coil pack. I have vacation coming
up and I think I'm gonna throw my spare intake
manifold on there with the wiring cause I'm fearing
that I may have fried some wiring before popping the
fusible link while dropping that alternator.


If anyone has any more ideas I'd really appreciate it
as this has been driving me crazy and I really dont want
to get rid of this car as I've had it over 3 years and I love it.

I know it wont hurt to do plugs and wires too, but they're only
about 6 months old. I cant see anything fouling them out that
fast, but do you think I should bother? I'm not really looking to
throw a bunch of money at it if it's something small as I've already
put about $300 towards fixing this.

[sqc151]

check your cats. a clogged cat will bog u down and make the car shake a bit.

[vrg3]

What did you accidentally drop on the radiator support? The alternator, with the battery cable attached to the post, and the post uncovered?

Popping a fusible link means pulling a lot of current. The heat could have burnt more than just the fusible link. Have you taken the fuse box apart to inspect everything? If you have access to one, I would just replace the entire fuse box if I were you. An unreliable power supply could plausibly make weird things happen intermittently.

When you pull codes, it shows you all the codes in memory. You don't have to do it while the CEL is illuminated. Pull the codes and let us know what you find.

It's probably worth at least inspecting the plug wires and making sure they're solidly connected. A loose plug wire could plausibly cause this weird kind of misfire/backfire.

[Legacy777]

Is your car AT or MT?

When you experience the shaking, if you push in the clutch if MT or put the gear selector in neutral does that change the shaking, even if you keep the engine rpms up?

I don't quite see how your shaking is related to the engine. It could be drivetrain related, but you need to do some more troubleshooting to see.

[SWRT]

sqc151: No cats left. Tried that first since it was free.

Vikash: Yeah. Dropped the alternator on the support with the battery
still connected. I was trying to get it done fast instead of right. Lesson learned.
I haven't taken the fuse box apart. Haven't had time yet. I may be able to find
a junk one, but the problem is finding one from the same car that hasn't had
the hood open for 3 years
Dang...I didnt know it saved the codes. Gonna have to try and do that here
in the next few days. I have Thursday and Friday off to try stuff.
Eh, I checked the plug wires and stuff. They look good, but I haven't
checked the plugs themselves since the swap.

Legacy777: It's an automatic. The thing is the trans has always
run nicely and the rear diff never gave me any trouble. I think
the shaking is the engine missing or something and it's just
rocking on the old motor mounts. But yeah, if I drop off the
gas pedal or put it in neutral or shut the car off the shaking
stops immediately.

[vrg3]

Hehe, yeah, I'm sure you'll never make the mistake of failing to disconnect the battery again!

Yeah, finding a replacement fuse box might be a challenge, but I honestly would probably never trust a fuse box that survived a blown fusible link. Seriously, it takes like 100 amps. That's like 1200 watts. That's the power rating of a microwave oven!

Car-part.com might be a useful resource. In case you're unfamiliar with the site, it basically indexes the inventory of a bunch of junkyards nationwide. For a lot of parts, they have interchange info, too, although not for Legacy fuse boxes (just specify a non-interchange search using only Subaru Legacy from 1990 to 1994). When you find some, you'll probably want to just call the yards on the phone; in my experience they're usually not very responsive online. A lot of the yards offer warranties, so you can expect their parts to be in decent shape.

[SWRT]

Right? I figured it was a pain because I have freakin 5 different
wires running up there for my foglights that are hard to line up.

Nah, my friend Joe uses car-part.com at his shop for hard to find
parts haha

How difficult is a fuse box to change out?

[SWRT]

Dang...the closest one with a fuse box is 47 miles away
I could try the one down the street. I know they'll have two,
but the hoods have been left open. Cant remember if the
lids are on or off, but I know people like to steal the fuses.

[vrg3]

You have fog light wires going to both the positive and negative terminals?

From what I can remember, a fuse box is pretty easy. It's held in with two bolts (one of which is shared by the washer tank, so you might need to remove that), and then there's this kind of snap-in plastic cover on the bottom that you remove to expose the connections, most of which you disconnect like any other harness connector. I think the one big wire from the battery ends in a ring terminal so you undo a nut for that one.

A fuse box left open to the elements might be a gamble, but if it's cheap and nearby, I guess you may as well try it. We don't even know for sure if the fuse box really has anything to do with your problem.

[Legacy777]

Legacy777: It's an automatic. The thing is the trans has always
run nicely and the rear diff never gave me any trouble. I think
the shaking is the engine missing or something and it's just
rocking on the old motor mounts. But yeah, if I drop off the
gas pedal or put it in neutral or shut the car off the shaking
stops immediately.

Maybe you need new motor mounts....

If they are really worn, there may be misalignment that's causing issues....who knows.

If you put the car in neutral and rev the engine while coasting, is the shaking still there?

[SWRT]

That's a good question. I'll have to try that. The motor
mounts looked ok when doing the engine swap. The motor
mounts that came out from my old engine were toast. One
and a half came out with the block. The other half of one
was still on the crossmember.

[SWRT]

You have fog light wires going to both the positive and negative terminals?

From what I can remember, a fuse box is pretty easy. It's held in with two bolts (one of which is shared by the washer tank, so you might need to remove that), and then there's this kind of snap-in plastic cover on the bottom that you remove to expose the connections, most of which you disconnect like any other harness connector. I think the one big wire from the battery ends in a ring terminal so you undo a nut for that one.

A fuse box left open to the elements might be a gamble, but if it's cheap and nearby, I guess you may as well try it. We don't even know for sure if the fuse box really has anything to do with your problem.

Nah, I got two extra cigarette lighter ports hooked up too.

Sounds easy enough.

Right, but like you said most places offer money back for
non-working parts and chances are it'll be less than $20 at
the junk yard down the street from work.

[SWRT]

Didnt get the chance to make it to the junk yard this weekend.
I'll have to try and go after work tomorrow. Then I need to clean
out my spare intake manifold and I think I'm gonna spray it with
wrinkle paint. Then I'll swap out the fuel rails from my old one and
hook up the electrical components on my new one. Hopefully that'll
fix something.

I did take the fuse box apart though and I didnt see any bad wiring.
Nothing looked burned or frayed.

I drove at 65 when the engine shook and I hit it into neutral
and it stopped immediately. I did notice that my passenger
side front shock is bad though and that I need a new ball joint
on that side.

[Legacy777]

It sounds like the vibration is drivetrain related. I'd definitely suggest checking your engine & trans mounts.

[SWRT]

Yeah, that I'll be doing when I get time. I have to throw
the car up in the air, but Florida is unreasonably cold for
some reason. I have to change out that passenger side
ball joint too.

Any reason why the engine would not let me over 55mph
for a while though? Why it just hits a wall and starts to
backfire?

[Legacy777]

Not sure about the reason you wouldn't be able to go over 55. If you put it in neutral can you rev the engine at all, or does it hit a wall as well?

[SWRT]

That's a good question. I haven't taken it above 55 in at least
a few weeks. I'll have to give that a shot. Like I said, I can
work through it and once it gets up above 60-something it
runs smooth with no issues. Just accelerates sloooowly.