You can test them, there'* a test in the tech info area. If there getting weak you wont get a strong spark so rough running and lower power.

So here'* an observation which might be critical in figuring this thing out. I just realized that the rough idle in park only happens when the car is on a hill facing up (where the hood is higher than the trunk). I went to the same place I went a few weeks ago where I first noticed it, and it did the exact same thing which got thinking about the select other places where it'* happened: all on hills! What could this mean?

Check your fuel pressure. Does it happen more with the tank half full or lower?

if i were you i would open your hood get in it put it in drive and give it some gas while applying the brake and observe how much the motor moves and you can also put it in reverse and do the same thing

It'* happened with all different levels of gas. And it runs fine in any other gear, just park it'* rough, and just on hills. Is there something about being in park that makes the car idle differently?

I've had all the motor mounts checked, they're fine. However, this engine was recently taken out for main/rod bearings so maybe something isn't installed correctly.

Have you checked your fuel pressure? Was the filter changed? Did you try injector cleaner? Have you checked and cleaned the throttle body? Have you checked your PCV and EGR? Checked your plugs, wires, and coils? If you have done all this, and still idling rough you should do a compression test. I've seen a engine pass all testes including a compression test only to find a broken valve spring.

There is the possibility that any of these things could cause a rough idle in general, but my problem is very specific: only in Park, only on hills, and the idle is not just rough, the engine is barely turning over while it varies from 900rpm to like 600 or less and jumps back again, all the while with a serious amount of vibration. Shifting into any other gear fixes the problem and the engine will idle smoothly. Because the issue is only in park, I can only imagine it'* a transmission issue, or something closely related.

Wow sounds similar to my 95 ssei, it make a knocking in park and such but other gears were ok, My current 96 lss shudders a lil when idling and it has a new intake, plugs and wires, so it could be just the wires or a fouling plug...

So you say your 95 did it, and then you also say RIP because the transmission blew. I hope the problems aren't related!

Okay this is weird, but I have to bring this up again. Now my car is doing this! Only in park, and it just started. Idles fine in any other gear, but the instant I put it in park it has a slight stumble to it.
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