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Hard starting and very rough idle at low rpm engine runs…

Customer Question
Hard starting and very...

Hard starting and very rough idle at low rpm engine runs fine under load

Mechanic's Assistant: Is the rough idle intermittent or consistent?

Consistant

Mechanic's Assistant: What is the model/year of your Volvo?

1999 s80

Mechanic's Assistant: Are you fixing your S80 yourself? What have you tried so far?

Didn't do anything yet

Submitted: 8 months ago.Category: Volvo
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Customer reply replied 8 months ago
The car did this one other time about a week ago my wife stopped to get gas a mile from home and when she started the car at the station it started and ran fine. Today we started the car and ran fine until we tried to start it leaving the grocery store. It started after about five minutes of on and off attempts to start it but run extremely rough at low rpm and fine under load
Answered in 1 hour by:
11/19/2017
Volvo Mechanic: Matt, Engineer replied 8 months ago
Matt
Matt, Engineer
Category: Volvo
Satisfied Customers: 23,306
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Verified

Hello
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.
1 vacuum leak path that you won't find with the above test is if the brake servo is leaking

so try clamping off servo hose to see if this has any effect

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

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Customer reply replied 8 months ago
I checked it out further and it was
Variable timing solenoid
Volvo Mechanic: Matt, Engineer replied 8 months ago

Understood - although this would have normally turned on the check engine light

is there anything else I can help with?

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