Hi everyone! My trusted Volvo mechanic is having great difficulty in fixing this problem for me. 1991 740 Base with LH system: It starts easily, runs well, very smooth on highway. Just a slight vibration at idle. You can feel engine "misfire" or "hiccup", or as if "one injector didn't squirt" or "had no spark one time", feels like it "bucks once or a couple times", that causes this interruption. Any speed, anytime, though seems only when car is warm. This is at highway speeds where best felt. It is not the AC cycling when I use it. I have chemically cleaned the fuel injectors. No codes from the on-board diagnostics module under hood. Mechanic has replaced: Pre-Pump, (Main has less than 30k miles on it), crank position sensor, Suppression relay, newer used distributor, new cap, rotor, and wires, New fuel pump relay, engine temp sensor, Ignition control unit. I am heartbroken spending all I have, but I really don't want to drive anything else! Love older Volvos!!!
He doesn't think it's any wiring issue, but I historically don't understand wires anyway and have refused to learn. I do not see any bad looking or worn, or missing insulation on any wires anywhere underneath or in engine compartment. I am bummed out and don't know what to do or when to consider "giving up". The engine is immaculate with no oil leaks and engine compartment is very clean! Anybody have an ideas what to do? Thank You most sincerely!

I just remember about my Saab 900s would miss-fire when the engine became warm. I'm pretty sure your problem is the hall-sensor, Use electronic freeze on the sensor when the missing begins to check the hall-sensor. I have a 1989 740 and not sure if the sensor is inside the distributor. I'll check tomorrow. Your car runs great before the engines warm' so it makes sense to check the Hall-Sensor.

EZ 115-117K Distributor has the hall sensor. EZ 102k distributor has an impulse coil

Hello To All the Brickboard Community!: I couldn't drive the 740 as it did some violent bucking on the Hiway-Disconnected the AMM and got home. Wiggled the wires on the Drivers strut housing and the vehicle died; did this a few times and felt now that the issue may be the wiring harness near AMM to under the manifold. Tried to duplicate same the next morning but could not, and limped it to the mechanic. I put in a used AMM and code erased. Car cut out on the way to mechanic. They didn't work w it last week so hope this week.
For consideration in my desperation, sometimes my ignition key sticks and causes the starter to continue after engine starts; you must manually turn key more counter-clockwise to turn it off, again not all the time. Do you think this may be a causative issue of the cutting out/misfire problem? Thank you to all-I will keep posting! Jeff

Well,I've had a new AMM installed and so far have 300 miles without ANY hesitations or misfires!!! It is still early for my level of trust to increase. I also do not understand how and why this was not remedied sooner by Volvo mechanic I've been using for over 20 years! I believe all the items I replaced will contribute to longevity, yes, but no wonder these vehicles last so lone as many times you see your mechanic and replace everything they see!!! Good Luck to all: I AM CONVINCED that part quality is not very good any more as well! Thank You to all who have helped me! Jeff

What the solution REALLY was, was that the wiring was bad either TO or AFTER the AMM, and intermittent shorting out the Mass Meter. My mechanic bypassed this to another set of wires, thereby fixing this issue!!!

Thank You! I will ask my Volvo mechanic!
My update so far is yesterday I put in a new coil, drove with 1/8 tank of gas south of town. Long straight aways where I could gun it, floor it, numerous times; up yo 85mph for many miles, some uphill. For 40 miles not one perceptible miss-fire!!! I turned around to go back home the same highway and BAM! had some miss-fires, but seemed not at 5k rpm 85mph. Seemed maybe 55-70 mph unpredictably, random cadence, no regularity...I'm aghast and agog! What is happening??? Jeff

Have you tried another coil? I don't see it mentioned in your laundry list of stuff you've replaced.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the brake fluid leak - now on to the ignition)

Thank you Chris: I actually thought of that yesterday after crying to this mechanic. I will check the wire first if that was included in his installing new plug wires! Will keep you posted, and Thank You!

Replace or re-solder the fuel pump relay (system relay, fuel injection relay) and check the socket for melting while your in there.

Additionally, make positively, absolutely sure that the coil to distributor wire is seated correctly and has enough slack. Not sure if you car was a effected, but the Bougicord wire set for my 1993 940 does not include a coil wire (sold seperately) and the OEM wire is just barely long enough for a car with NEW motor mounts (mine is a head-mounted distributor).

Thank You Onkel:I will check today about the coil wire; Mechanic installed new Fuel Injection Relay. By "System Relay" do you mean the rectangular FI module under the drivers knees? I provided them with a good used one and it didn't make a diff either. Thank You: Iwill keep you posted!

Have you replaced the fuel filter? When my 740t started missing, I found that the fuel pump relay needed to be re-soldered. These would be the next two things to check. Do a sweep check with an Ohm meter on the throttle switch. Any dead spots inside the needle sweep contact area will also cause missing.

There can be many names for the same relay depending on year and the company describing it. It is two relays in one on most late 240's and all 940's so I assume it is the same on the late 740's. To be honest, not even sure wher it is on the 740 but I assume the fuse and relay tray is the same as a 940 behind the ashtray.

Do you even have the head-mounted distributor (back of the head, big orange thing)?

Keep in mind, if you mechanic does not specialize in Vovlo's, he may not be away of the "unique" issues with various aftermarket parts. some are better than OEM but most fall well short.

This is almost certainly an electrical issue focused around heat. The RSR (also a relay but under the hood) is common issue with these cars.