Rough running 1990 Legacy

[Brutewrench]

Hi all, I have been fixing up a 1990 Legacy wagon, automatic. I have overcome most of the major challenges (cold solder joints on the heater control causing intermittent operation, noisy front wheel bearing, and a leaky radiator), but I am stumped by a rough running engine.

 

The car idles really rough (shakes the car). It has good power when accelerating hard, but if driving along around 2000 rpm, it starts to buck as if there is crossfire. It feels like it is going to die, but a sligh increase in throttle and speed eliminates the problem.

 

The plugs and wires were replaced by the previous owner just before I bought the car. The check engine light has not come on since I have had the car, but the records indicate that some sensors have been replaced in the past. The O2 sensor appears to be original. I could not detect any vacuum leaks in the engine and the PCV valve is fine.

 

A couple of other possible clues - The car had overheated in the past, due to thermostat problems and the radiator leak. All the plugs were clean, but one plug did have a much darler insulator than the rest.

 

Any tips are appreciated.

 

Thanks,

[Eljefe]

Hmmmmm... Overheated ? I'd look at a head gasket. You could do a leak down test.

[etlsport]

I had the same problem in my 97 legacy... valves in the 4th cylinder were corroded and had to be replaced to the tune of $700... what a bummer

[Brutewrench]

Thanks for your replies!

 

Well, I had ruled out a blown head gasket because the antifreeze is just antifeeze and the oil is just oil. It also does not make any funny noises that I have come to associate with head gasket failure in other cars.

 

I had considered a burned valve, but was hoping to hear that burned valves never occur. The fact that one plug was dark could lend credance to a burned valve. I guess I'll try a compression test - I suspect it wont show up unless it is really bad.

 

I guess neither option is particularly pleasant. Does the engine need to be removed from the car to pull a head? Also, are the blocks still prone to stripping threads? The last head I pulled was on an 84 Gl and I ended up having to put inserts in the block after the threads pulled out when removing the head bolts.

[Markus]

Whoa....you just described most of the problem I have ('90 Legacy Loping Idle Thread) without the known previous overheat and darker plugs. All my old plugs looked the same.

 

I'll post any additional findings in my other thread.

 

My shaking car rough idle started on a damp day just like a shorted sensor or something but I have done/checked a lot (listed in thread) but have not fixed it yet.