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95 850 Turbo with Extreme Engine Surging

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

Nick777
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Joined: Thu Oct 24, 2019 7:40 pm
Year and Model: 1995 850
Location: New Jersey
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Nick777

95 850 Turbo with Extreme Engine Surging

Post by Nick777 » Tue Jan 14, 2020 10:05 pm

Ok, so the symptom is that my car can be idling normally, or my engine can be responding predictably (meaning more throttle results in higher rpms). However,....

It often occurs, both while idle as well as while driving, that my engine rpms surge... and it some cases i mean SURGE (from idle to 5000 rpms has occurred, though not usually this bad). As you might imagine this is potentially incredibly dangerous if it occurs while driving. Anyway,...

I KNOW that the Coolant Temp sensor (or its connection) is bad. Now i know that's confusing the fuel injection system, but i dont imagine that alone is causing this crazy surging issue.

Here are my thoughts:

1) Throttle leakage/sticking - don't think so based on symptoms appearing without playing with throttle.
2) IAC issues - don't think so because, really, can an IAC even open up enough to allow sufficient airflow to cause surging from idle to 5k+ rpms?
3) Vacuum leak(s) somewhere? I dont think so because isnt the only fuel injection "modulator" downstream of the MAF the pre-cat O2 sensor? And does that O2 sensor even have the capacity to modulate fuel delivery enough to send a normal idle to 5k+ rpms?

Sooooo.... my thoughts run along the lines of this --->

1) there MUST be leaking air coming in from somewhere, possibly from multiple locations, or else the engine couldn't possibly ever surge as high as it does and,...
2) there MUST be some involvement with a major sensor involved in modulating fuel delivery (not the O2 sensor, but i suppose that could be bad as well), like the MAF and TPS. Of course, there is the ECT sensor issue, which we know about already.
3) possible partial involvement of *Multiple* issues of the types stated above. Even if one of those above stated possibilities alone can't cause the surging, probably if multiple issues are contributing, they could.

If you've made it this far, thank you for your patience!!

I'd love to read your thoughts!

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erikv11
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Year and Model: 850, S/V70, 06 S60R
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erikv11

Re: 95 850 Turbo with Extreme Engine Surging

Post by erikv11 » Thu Jan 16, 2020 1:14 pm

I would

(1) Fix the ECT. Go junkyard if you don't want to pony up for the OEM one.
(2) Check everywhere for vacuum leaks.
(3) Unplug the MAF, start it up, see how it goes with the MAF unplugged.
(4) Clean the throttle body if/when I could talk myself into it. Clean the IAC at the same time.

Does this car have OBD2? Can you hook up a reader and get data while it is surging?
'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, 285k
'06 S60 R, 160k
gone: '96 NA 850 210k, '98 NA V70 182k

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