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1998 V70R occasional misfire (P0300)

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Year and Model: 1998 V70R
Location: Massachusetts

1998 V70R occasional misfire (P0300)

Post by uyvolvo »


I have a random misfiring at idle that is driving me and mechanic nuts. Details:

1998 V70 R, 165 K
Symptoms: car will start fine, run for a while and then occasionally misfire at idle or at stop-lights. Throws a P0300 code. Tachometer stays steady at 1000 RPM and car will usually run well after these misfirings It also runs well at higher speeds.
Did a full tune up (cap/rotor/spark-plugs/wires), changed crankshaft position sensor, knock sensor, coil. Nothing helps.
Mechanic thinks that it may be a weak valve spring and suggests doing the heads ($$) but I am not convinced, since I thought that a weak spring will lead to poor running at higher rpms, not idle.

Any suggestions / help greatly appreciated.
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Re: 1998 V70R occasional misfire (P0300)

Post by WhatAmIDoing »

Grab a new Volvo fuel pump relay. I had a KAE relay cause random misfires at idle as it began to fail (didn't realize it was the relay until after I did new plugs, wires, cap, & rotor). Consider changing the fuel filter as well if you have no idea when it was changed.

You should look for vacuum leaks. Your idle should be closer to 800 RPM, not 1000.
'98 S70 T5M - 282,000+mi - forever a project
'99 S70 "AWD" - 220,000+mi - part out :cry:
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Re: 1998 V70R occasional misfire (P0300)

Post by misha »

Check gap on those new spark plug(if they are not Volvo plugs),since it's critical for turbo engines.
'97 850 2.5 20v / fully equipped / Motronic 4.4 from the factory / upgraded with S,V,C,XC70 instrument cluster / polar white wagon
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Re: 1998 V70R occasional misfire (P0300)

Post by scot850 »

We say it time and time again that the parts used can determine issues you may get. Can you state the brands of the parts you fitted as some brands are known to be incompatible with these turbo motors.

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Re: 1998 V70R occasional misfire (P0300)

Post by erikv11 »

Agreed, super important: replaced ignition components with what brands of parts?

And I like the fuel pump relay idea as one possibility, and note that if the misfires are consistent you can test it with a jumper wire before you shell out the $45 for a genuine Volvo relay.
'95 854 T5-R, Motronic 4.4, 185k
'96 855 NA, 145k
'98 S70 NA, 220k (living out west)
'98 V70, T5 tune-injectors-turbo, LPT engine, 293k
'06 S60 R, 167k
'07 XC70, 170k
'99 Camry V6 :shock: 130k
gone: '96 NA 850 210k, '98 NA V70 182k
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