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Starting fluid can damage diesel engines that use glow plugs. What ever damage may have been done is done. You need to find out why the PO needed to use starting fluid. Discontinue the use of starting fluid. If you need to, use WD-40 in place of starting fluid.

Get a factory service manual for that car. It can be found on CD inexpensively.

Remove all the glow plugs. Test them.

Perform a compression test. Compare the results to the FSM specifications.

Test values with engine at operating temp. approx. 80 C
Compression pressure, normal 26 - 32 bar
Minimum pressure, approx. 18 bar
Permissible difference between cylinders max. 3 bar

If the compression is good and the plugs were bad, easy, cheap fix.
If the plugs are good and the compression is bad, not so easy or cheap.

Was the car ever run on alternate fuels ?
Is there a lot of blow by ?
Smoke in the exhaust ? what color ? when ?

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO