1991 740 non turbo starts hard, bogs, sputters, front-fires

dnarby · Post by **dnarby** » 25 Jun 2010, 18:47

Hi guys,

Glad to have found this forum. I checked the error codes via https://www.matthewsvolvosite.com/forums ... =2&t=24944 and found no errors except ones I expected (see below, and BTW the repair database is missing some of the codes on that list, but I'm really glad someone posted the PDF file, that's awesome to have).

My 1991 740 non turbo starts hard, bogs, sputters, front-fires (backfires out the air intake tube), and will stall under full throttle (in park). If I baby it it will rev up OK. If I squirt some gasoline in the intake tube while revving it, full throttle doesn't bog it down.

It started hard before this, but ran fine once warm. Recently started bogging down under full throttle. Bad enough now that I don't feel comfortable driving it.

Replaced plugs and wires (looked serviceable, but I bought it used, so who knows), no joy. Have not replaced distributor and rotor (looks serviceable, but due for a change).

Disconnecting the air temp sensor didn't change anything (but it did register 1-2-2 on the fuel self diagnostic, which is good to know). It also threw 3-1-1 on the fuel diagnostic, but my speedometer is intermittent (I use my GPS for a speedo). Ignition diagnostics threw 1-1-1.

Was told by a mechanic to disconnect the MAF and see if it changed anything. I cannot locate the MAF sensor! The air temp (charge?) sensor is readily visible (first sensor after the air box), but there are no other sensors in the tube up to the manifold, and I see nothing that resembles pictures of the MAF from Rockauto.

Halp!

TIA,

Dave

Post by **billofdurham** » 26 Jun 2010, 11:57

Welcome to Matthew's Volvo Site.

The MAF is mounted on the fresh air intake hose, near the air filter as in the attached.

740 Engine sensors.pdf: (351.49 KiB) Downloaded 744 times

The intermittent speedometer signal could be causing your problems. The speed sensor not only operates the speedometer it is the crankshaft sensor. If it is giving spurious signals to the ECU you will have problems. The sensor is easy to change and relatively inexpensive.

Bill.

dnarby · Post by **dnarby** » 26 Jun 2010, 14:34

Thanks, I'm really glad I found this site, and many thanks for offering to help! (I hope) you're gonna love this...

My engine appears not to have a MAF(!).

It does have an Air Charge Temperature Sensor sensor where the MAF should be, view of down the tube directly after the air box

The sensor in the tube looks exactly like the picture at Rockauto.

Overhead view of the engine (sans distributor, pulled to inspect it)

According to the various under the hood and door stickers, it is a B230F engine, Family NVV 2.3 V5 FR8X, evaporative family E3, ex emission control system MPI, HO2S, TWC, the car was manufactured in Belgium on 09/91.

Retired MVS Contributor · Post by **Retired MVS Contributor** » 26 Jun 2010, 15:30

The MAF is part of LH-Jetronic system...Your Volvo has the Regina system and there is no MAF...You have already checked the air temperature sensor...On the left fire-wall-to-spring tower brace you will see the diagnostic unit...Right next to it is the pressure sensor...The pressure sensor has a vacuum hose connected to it and the symptoms you are getting sounds like the hose is off...If it is not, then the unit should be checked...

dnarby · Post by **dnarby** » 27 Jun 2010, 18:24

Hi Jerry,

Thanks, at least now I know a bit more about this wagon.

On the brace to the spring tower there is a doo-dad which has a white semi-opaque hard plastic vacuum line going into it and also a 4 wire flat electrical connector (upper right corner of the picture I took, unit has a gray metal shroud with two Phillips head screws holding it to he strut, line and connector are on the underside hidden from view). Is that the unit, and if so, how do I check it?

dnarby · Post by **dnarby** » 27 Jun 2010, 21:59

Sorry, forgot to mention that (unless I missed one, but I took care to be thorough) all vacuum hoses were attached, and appeared to be servicable.

Also, not wanting to get ahead of you, but is this the sensor in question?

http://www.volvoclub.org.uk/faq/EngineS ... sureSensor

Thanks in advance,

Dave

Retired MVS Contributor · Post by **Retired MVS Contributor** » 28 Jun 2010, 07:09

Yes, that is the pressure sensor...Did it throw a 1-2-1 code when disconnected?...Don't forget yourself and leave the ignition turned on when you are disconnecting things...

The link you included tells you how to test both the temperature and the pressure sensors...For the temperature sensor you need an ohm meter and a heat gun...For the pressure sensor you need a vacuum source such as a Mityvac hand pump...The pressure sensor is a piezoelectric crystal and it's output is modulated by vacuum...

There is a more detailed test procedure for the pressure sensor available (Vadis) which involves accessing the control unit in the passenger's kick panel and taking voltage readings as the vacuum is altered...

dnarby · Post by **dnarby** » 29 Jun 2010, 13:28

Hi Jerry,

Tested both sensors, both appear to be operating normally. Vacuum line to pressure sensor seems OK.

It did not throw a 1-2-1 code when I checked the OBD with the pressure sensor connector disconnected. Car died when I pulled the vacuum line to the pressure sensor.

One other note: If I give it full throttle from idle, it dies. If I give it a tiny bit of gas and rev it up to ~3k RPM and then give it full throttle, it winds up fine.

I don't know if this is possible, because it didn't throw an O2 sensor code, but I read a bit more, and am wondering if it has a bad O2 sensor... Previous owner had replaced the head which had warped. Per that page earlier "Antifreeze contaminates oxygen sensors. Oxygen sensor replacement is highly recommended after replacing cracked cylinder heads, leaking intake gaskets or leaking cylinder head gaskets." Pretty sure they didn't replace the O2 sensor, I drove it for several months after this repair had been done.

Thanks,

Dave

dnarby · Post by **dnarby** » 29 Jun 2010, 17:02

This sounds exactly like the problem I have...

"[Diagnosis 6:] Oxygen Sensor malfunction. If the engine bogs and will not rev, with a very slight hint of backfire/misfire, while accelerating from a dead stop; or cracks, once in a while, like it is backfiring through the intake, then check the oxygen sensor. Several correspondents have reported sensor overvoltage (a peak output of 1.3V instead of the normal 0.9) from generic sensors (Bosch, NGK) even if relatively new. Replacing the sensor solved the problem. "

From http://www.volvoclub.org.uk/faq/EngineP ... iagnostics

Can an O2 sensor go bad w/o throwing a code?!

dnarby · Post by **dnarby** » 02 Jul 2010, 22:46