'92 740 b230ft strange idle and throttle problem

[emoreno619]

hi all,

I have a '92 740 b230ft with 120k miles. It is has been my daily driver for the last two years with few issues, and has been well maintained (e.g., regular oil changes, hose repair, changed water pump and radiator, etc.). Today while driving it seemed to choke on gas, not accelerating well, and almost stalled. But it got me home, and at rest the idle seemed a bit high (~1100rpm).

I have done some research and understand that these issues can be related to either a.) a faulty MAF (mass air flow sensor or AMM, air mass meter), b.) a dirty or faulty throttle plate/body/idle air control valve, or c.) an failing ECU.

To diagnose, I tried unplugging the MAF sensor and this appeared to cure the high idle. However, when I drove with the MAF unplugged, I still had throttle and acceleration problems. My question is: if the MAF were the culprit (as it seems to be since my idle returned to normal without the MAF), shouldn't I be able to drive normally without it? Why not? Will replacing the MAF fix the high idle AND throttle issue?

Or are these two separate problems surfacing at once? If the throttle/acceleration issue is separate, do I begin by cleaning the throttle body and plate? How do I know whether it is functioning correctly? Where is the idle air control valve?

Though I'd prefer not to, can I drive it? Or will I further damage the car and risk stalling on the road?

Thanks for any help!

[98T5]

Do these things:

1. Claen throttle body
2. clean TPS
3. clean IAC valve
4. clean injectors
5. check motor mounts
6. check tranny mount
7. check/repelace O2 sensor
8. change spark plugs/cap/rotor

^Most causes of rough idle

^^so far leaft I have to clean my injectors, replace O2 sensor and update my motor mounts. g'luck

[Volvo740FAN]

Idle control valve is located below the intake manifold. You can access it from the left side of the car: 1. open completely the hood (I guess you know how to make the bonnet go more than usual i.e. under 90 degrees up - move little metal plate on both hinges down (or up, I forgot)

2. locate the valve: you'll see two rubber hoses hoses coming into one small cylindrical aluminum part

3. electrical connector is almost invisible, it is on the firewall side of the idle control valve.

4. disconnect that connector with your right arm, most likely you will have to do that by feeling

5. assess the difference

6. remove the valve and clean it - use contact spray or WD 40. For cleaning arrange the bypas between two hoses, see if you can somehow connect the idle control valve to its connector (without the hoses), and start the car. Spray WD 40 inside, leave it few seconds, shake and throw away the liquid (think of the environment, collect chemicals, even WD40, into something that will be recyclable)_

My experience with these cars is that this idle control valve is the main culprit for idling problems. This idle control valve is in essence an electromotor controlled by your lambda sond. Usually, it causes high rpms and keeping high rpms after eactivating gas pedal. If a car was sitting long, more chances that idle control valve creates problem. As it is controlled by the computer/lambda sond, its correct finctioning is essential for all systems i.e. petrol consumption.


Another problem that can cause rough /choke like running and idling problems and excessive use of fuel is broken membrane on the fuel pressure regulator: small metal filter-like thing on the rail that supplies fuel to injectors. A small diameter rubber hose goes into the throttle body: remove that hose while engine is running and see if there will be drops or drips of petrol. It must be totally dry if working correctly. If the mebrane is broken, additional fuel will enter the inteake manifold and that won't be good.,,,

[98T5]

Good info Volvo740FAN

I'm having a rougher idle since yesterday and I think the fuel pressure regulator is my issue. I cleaned my IAC already and it's working fine. so this regulator could be my gremlin. my 940 was owned by an old lady and I'm sure it sat for long periods of time.

[emoreno619]

Cleaning the MAF appears to have solved the problem. I am at least able to drive and accelerate normally. The idle has returned to normal (a little less than 1000, ~750-850 RPM). I'm not sure the fuel pressure regulator is responsible since I changed that somewhat recently.

A couple of observations and questions:

First, although the idle has returned to normal, the tachometer still bounces every so slightly. Is this normal, or an indication of a failing MAF, or something else?

Second, now the check engine light is on. It has never come on before. Can I use the OBD to determine what is triggering the light? Do I need a tool or cable to use the OBD?

Third, I believe the MAF may have been dysfunctional because the foam padding that sits directly above the air filter near the MAF had begun to disintegrate. The foam didn't appear to serve any real purpose and to avoid having to clean the MAF soon, I simply removed all of the foam to prevent it from contaminating the MAF again. Is the car okay minus the foam?

Finally, should I still clean the IAC valve and throttle? How about checking/cleaning the O2 sensor? How can the sensor be tested?

In general, I am fan of: don't fix it if it isn't broken. However, I would like to preempt future fuel system issues since they may currently be hindering my gas mileage. Is it worth risking damage to possibly already functioning parts in order to test them?

Thanks again for your advice. It is invaluable, and the strength of the volvo community online is a large reason why I bought my 740!

[98T5]

Here's what I just did 30 minutes ago to correct my drooping idle. I simply adjusted the TPS sensor screw and the idle raced up to 2000 rpm at first and when I put the car in gear it went down to 1000rpm but the idle was smooth and consistent. So I figured I may have adjusted it too much until I took it for a spin....and WOW!!! I can feel a BIG difference in powerband and giddy-up and as I stopped and started from redlights the idle finally settled at 750-800rpm which was WAY better than jumping from 200rpm to 500rpm and back down to 300rpm and up again to 700rpm. such a annoying thing!!! ...but now.. the idle is smooth at 750-800rpm in gear with my foot on the brake and it holds at 1000rpm steady when the I shift the car into park. It's like the smallest Torx bit ever, but there is room for adjustment on the side of the TPS sensor bracket. I adjusted mine all the way out, which was only about 1mm. but it fixed my rattling idle!!!!

So if you are still having idle issues I would definitely try adusting the TPS sensor screw as part of tracking down the problem