Fast idle, poor fuel economy and hard "miss"

[AndyB]

The car has suffered from a poor idle for a while and can normally restore by cleaning the idle valve. Recently the idle speed as risen to ~1400rpm and the fuel economy has dropped markedly from >47 mpg to <37 mpg. The engine feels slightly "off" and has a hard miss/stutter if I accelerate moderately at ~3000rpm which is very repeatable, it also seem to stutter when slowing down with foot off the accelerator. I saw something on these boards before about the ECU effectively running richer and richer and changing the timing to the point when a hard miss would occur. Any ideas? I am nervous that if I run with it very rich I could do damage to the catalytic converter among other expensive hard to replace items.

I am in the US so the garage has no clue and just comes up with comments about idle valve etc.

Any thoughts?

VR
AndyB

[jamesy12345]

...and which kind of car is this? Can't be a Volvo 480 with those sorts of mpgs. Maybe American gallons.....!

Sounds like running rich. My guess would be the lambda sensor

[glasgowjim]

Andy as Jamesy says what is the model of your car..

volvo 480 ...

1.7 OR 1.7 turbo OR 2ltr engine

and what is the year of manufacture ?

[AndyB]

It's a 1.7 non turbo , 1991 (J) Paris. Mistyped miss speed corrected to 3000rpm

[Van]

I've had problems with idle and decreased economy which would get really bad when turning on the air conditioning. Turned out to be due to two things: one wire being loose/off/disconnected and some bad sensor. That would cause the system to run in emergency program. I'm sorry I can't name the wire and the sensor.

Your idle being up in rpm could be by the system compensating for lack of fuel or air, or at least the system being reported so.

[glasgowjim]

Right Andy this is going to be a process of elimination unless some other member comes up with an idea or photo of their engine / wiring or whatever.
Still by sheer luck my own car is a 1992 1.7 NA engine as well (its even P\aris Blue)

right apologies if you have already done this but we have to know / eliminate all the options.

First remove distributor cap and clean up all the contacts including the rotor arm. then remove and clean the spark plugs . If they are very black/sooted up then you are definately running too rich.

check that the spark plugs rubber caps on the distributor leads are ok as I had a running problem and it was partly caused by sparks jumping from side of plug to the engine body.

then to check for misfire look at the wires onto the sensor thats next to plug one (the nearest to the distributor) my daughter had a misfire and it was caused by a broken wire to that sensor making intermittent contact.

the next obvious and easy check is to look at the intercooler the silver or black part at the top of the engine that has volvo stamped on it . Stnading at the front of the car look along to the left and their will either be 2 or three outlet connections (if you have airconditioning it will have 3 . none of these connections should vent to the air . My car has only 2 pipe connections the one nearest the front is sealed with a push on black rubber seal the second one has a small bore air pipe running from it.

try these first and let us know what the outcome is in each case.

I had a persistent problem that developed quite early on , at start up the car would go to 2000rpm then drop down to about 1400 rpm or sometimes to 1200rpm when warmed up . thats way too high I eliminated every option including lambda sensor and then replaced the engine ecu unit and my problem was solved. If you have to replace the engine ecu unit its only about £20 in uk money .

[glasgowjim]

Oh Van need some better indication as to location etc if you can .

Andy do a search on Van's posts he will have a thread on this .

[AndyB]

Jim,
I finally got back from holiday and went through the diagnostics, the spark plugs were definitely black. I am going to look through Van's posts now. I must admit I am amazed the ECU is down to 20 pounds. When I had the car at first the ECU went out and it cost 480 to replace it!

AndyB

[glasgowjim]

Hi Andy thats the second hand or used parts going rate.

CEM 3 units are about £30

VEM units can be anything from £10 to £30 (these are aircon)

Thats UK second hand prices, everything is a lot dearer in continental Europe.

ABS controller units second hand £20 to £30

and so on......

Oops price has risen to £25 and you can get it from ...

http://www.ebay.co.uk/itm/GENUINE-VOLVO ... 1399wt_758

that is who I purchased from. If he only delivers to UK addresses you can always get it sent to me and I will post on.