1994 940 NA Rough Idle After Head Rebuild

TacoSalad · Post by **TacoSalad** » 17 Mar 2015, 19:01

I just finished rebuilding the head on my 940 (with the B230FD) to fix a broken exhaust head - this meant removing everything including the valves and camshaft. I replaced all the various gaskets and seals as well as timing belt and tensioner. Now that i have everything back together the engine runs great except it idles very rough - about 500-600 rpm and very unevenly. It makes a ton of noise and vibration. The moment i touch the gas pedal at all it runs perfectly and driving around I have no problems except when idling.
Now before I pulled the head I had just cleaned the IAC valve and throttle body and replaced the throttle body gasket and it idled fine. I've checked for vacuum leaks and haven't found anything obvious, and there are no codes on the diagnostic port. Any ideas of what to check next?

Edit: Also I've recently replaced the spark plugs and wires as well as distributor cap and rotor.

93Regina · Post by **93Regina** » 17 Mar 2015, 20:41

TacoSalad wrote:I have no problems except when idling...

Word for the Day - Pulse air system B230FD

If sucking exhaust fumes into intake, there will be an issue.

TacoSalad · Post by **TacoSalad** » 17 Mar 2015, 21:30

Wow, that is a fantastic answer. On closer inspection the rubber y-tube that connects the two check valves to the vacuum valve is ripped right open. Now at this point I could buy replacement parts but since where I live there are no inspections emissions or otherwise I'd rather delete it if that's possible. Does anyone have any insight into how to do this? The holes in the head aren't threaded so I'm not sure if an off-the-shelf plug would work there. Can I unscrew the check valves and put a plug there? Or should I just deal with it an repair the pulseair system?

93Regina · Post by **93Regina** » 17 Mar 2015, 22:19

TacoSalad wrote:Wow, that is a fantastic answer.

Based upon what someone said in cited URL....

Volvo says, "This system adds air to the hot exhaust gases as they are expelled from the engine. This causes a secondary combustion of residual hydrocarbons and carbon monoxide, resulting in lower emissions levels in the exhaust gases."

However, if you have

Exhaust Gas Recirculation (EGR) - This system operates by returning some of the exhaust gases to the engine to be recombusted: since this lowers the combustion temperature the amount of nitrogen oxides released into the atmosphere is reduced.

and the EGR is passing exhaust to intake, then idle will be messed up.

I'm not familiar with this system, at idle, is there a vacuum feeding EGR valve?

TacoSalad · Post by **TacoSalad** » 17 Mar 2015, 23:19

My EGR system has been bypassed. The hole on the intake manifold has been plugged and the EGR solenoid has been unplugged. I did this before I removed the head to no ill effect.

93Regina · Post by **93Regina** » 18 Mar 2015, 07:27

Alexandrrr wrote:Volvo S80 ....getting a rough idle on cold start

Rough idle may point to a lean mixture:

1. Check fuel rail's pressure - sometimes, an older fuel pump is not up to full speed when cold.

2. Coolant temperature sensor - Check ohm reading when cold

3. Small air leak - Vacuum hoses or brake booster

4. Fuel Filter - When cold, a dirty filter will not allow as much fuel thru it.

>Volvo S80 Forum

I'm not familiar with this model's EFI system

93Regina · Post by **93Regina** » 18 Mar 2015, 07:35

TacoSalad wrote:My EGR system has been bypassed..

Idle Switch? Correct throttle plate spec?

Do it, by the book: LH2.4 Manual Complete

After engine is warmed up, pull IAC connector, then start vehicle...what is base RPM?

TacoSalad · Post by **TacoSalad** » 19 Mar 2015, 17:29

Thanks for all the ideas of what to check. Here's what I found out:

A fuel pressure test showed correct fuel pressure, so I think the pump, filter, and regulator are ok.

I warmed up the car and then unplugged the IAC valve and started it and - surprise - it now idles perfectly. I also performed the Haynes manual test where you pinch off the IAC valve bypass hose and the rpm dropped to about 500 which suggests the throttle plate is adjusted properly. So does this suggest that the IAC valve is just bad? When I was cleaning it I gave it a long soak in soapy water and I'm now thinking that this might have damaged some part of the interals.

93Regina · Post by **93Regina** » 19 Mar 2015, 19:20

TacoSalad wrote:unplugged the IAC valve and started it and - surprise - it now idles perfectly. .

PDF Page 17 - When the current is off, a spring sets the idle valve opening for an idle speed between 1,000 and 1,100 rpm.

LH2.4 Manual Complete - Your throttle plate must be adjusted via book setting, page 35. Your throttle switch (TPS) must be set via book specs...No air leaks...