91 740 Stalls when it gets warmed up then won't start until

Antaeus · Post by **Antaeus** » 08 May 2008, 16:28

Please help. My 740 stalls everytime it gets warmed up and then will not start again until it cools down. It now idles rough and backfires when going downhill with foot off of the accelerator. I have read for hours trying to find this same problem. I have put new plugs and wires on, replaced the knock sensor and the fuel pressure regulator. I have also checked for vacume leaks. If anyone can help, I would really appreciate it. Thanks!

Post by **billofdurham** » 09 May 2008, 11:47

Welcome to Matthew's Volvo Site.

Your car is fitted with a self diagnostic unit. Have you checked for fault codes?

A 'how to' can be found at: https://www.matthewsvolvosite.com/chat/p ... c&start=12

Although headed for '92 740 turbo the basics are the same. The codes may be slightly different, but if they are post them for an explanation.

Bill.

Post by **jonesg** » 09 May 2008, 16:24

Just taking a guess, the code will be O2 sensor related but it will be the cat converter. The insides of the coverter broke loose and slide fwd going downhill causing it to choke.

Just a guess til you pull the codes.

Antaeus · Post by **Antaeus** » 10 May 2008, 09:37

No error codes at all. I also tried disconecting the air temp sensor which did not help anything. I don't know what to try next, please help. Thanks.

Antaeus · Post by **Antaeus** » 10 May 2008, 09:43

Also, the stalling and no warm start occur everytime the engine gets fully warmed up (the temp gage is straight up not in the red or even close). Going down hill only sometimes causes backfiring. It has done the occasional backfiring going downhill for over a year. It seems to be related to the engine being pulled by the transmission because it does not backfire when going downhill in neutral. It was just a minor annoyance. I mentioned it only as additional info. The stalling & no warm start problem just started happening a couple of weeks ago. I have been trying to figure the problem out ever since. Thanks.

Post by **billofdurham** » 10 May 2008, 12:13

Pity about no codes so we go back to basics.

Things to check for No Start - Warm Engine:

Ensure fuel pressure is correct.
Ensure fuel injectors are not leaking.
Check ignition and sensor wires for proper connection.
Ensure spark plugs are not fouled.
Check ignition and coil circuits.
Check engine speed sensor.
Check throttle switch and connector.
Check idle valve and ensure airflow arrow points in correct direction.
Check for petrol in the oil.

Things to check for Backfiring:

Ensure ignition timing is correct.
Ensure timing belt has not jumped.
Check distributor.
Ensure fuel injectors are not leaking.
Ensure spark plugs fire with a strong blue/white spark.
Check ignition and coil circuit.
Check engine speed sensor.
Check throttle switch and connector.
Check exhaust system.
Check for petrol in the oil.

You may already have done some of these checks but they are out of my very old note book and I just rattle them off.

Bill.

lucast · Post by **lucast** » 11 May 2008, 06:38

Antaeus wrote:No error codes at all. I also tried disconecting the air temp sensor which did not help anything. I don't know what to try next, please help. Thanks.

On my 1989 740, I fixed my infuriating stall-when-warm issue by putting iin a new distributor with a new Hall sensor. But this problem was also confirmed by the varying rpm at idle.

Now have you checked the ignition module and the fuel injection/noise suppression relay? Stall when warm issues could also be due to problems associated with these components.

The ignition module is next to the battery (flat, black trapezoid shaped thing with a connector) attached to the fender with two screws.

An old ignition module may lose contact with the chassis which acts as a giant heatsink if the screws become loose. If the contact fails or the heat conductive compound (the white stuff) deteriorates, the ignition module overheats when the car runs for a while and then ceases to work.

On my 89 740, the two screws securing the base of the module to the chassis weren't too tight and the thermal compound was pretty dried up. I used slightly larger screws and leftover thermal paste from a DIY computer job when I refitted a new ignition module.