Logo

1998 57 Bad stumble

jws

New member
"This is a Volvo problem so pl

"This is a Volvo problem so please don't flame me, but I hoped some of you Merc guys might have experienced the same thing. (I also posted on the Volvo board) This has started recently, but is VERY intermittent. At low rpm (1000-2000 or so) it will run fine, and then suddenly feel like it almost stalls. It's more of a severe stumble that lasts a second or so, then will run OK again. Sometimes the stumbles will come only a few seconds apart, sometimes it will stumble a couple times then run fine for a long time.
From slow speed trying to get on plane, it will stumble a couple times then catch and run OK. Feels like a bad carburetor accelerator pump would feel, except its fuel injected, so that's not it. New plugs, rotor, cap, none of which helped. Some of the plugs were a bit wet, but nothing real alarming. Any ideas?"
 
Test the TPS or Throttle Posit

Test the TPS or Throttle Position Sensor with a DVM with ignition on in run position of keyed switch but engine off. Voltage should change smoothly from closed to open throttle. If the voltage suddenly drops and comes back up or "STUMBLES" you have found the problem.

Note: It can also be tested with a digital ohm meter with ignition off and harness disconnected. Ohms reading should change in a smooth progression without a dip in the reading.
 
"Guy,
Thanks for the input...


"Guy,
Thanks for the input...I'll check that, but it sounds like that would help me find the culprit if it stumbled only on acceleration...my main trouble is that it will do so after running anywhere from a few minutes to 10 or 15 minutes at low rpm (lots of no-wake manateee zones here)and then out of the blue it will stumble like it's trying to stall...no change in throttle setting or anything. The fuel filter/water separator is new as Bondo suggested, and there was no trace of water or other contaminants. My guess is an electrical / electronic problem, but I'm not sure where to start looking or what would typically cause this condition."
 
""My guess is an electrica

""My guess is an electrical / electronic problem"

Take your DVM with you and check the TPS when it is HOT since heat causes expansion. The TPS is only a potentiometer (variable resistor) that will wear out eventually. There may be nothing wrong with it or it may be saying "I on my last leg". Have you ever had a car radio that its speaker put out static when you touched the volume control or hit a bump in the road? Same thing as a TPS only the TPS is of higher quality.

All you can do right now is search for gremlins. I would start by REMOVING and scrutinizing each electrical connector. Begin with the main engine "barrel plug". Look for corrosion, loose, darkened or burned connector pins (arcing). Pull on the wires of the connector while observing the pins. Use your thumb or finger and push inward on male pins to check for movement. If one of the pins is sitting just a little deeper in the connector, grab it with a small needlenose pliers and see if it will move in or out. Any movement of the pins is suspect.

You may have inspected the computer connector but take another CLOSER LOOK; I have used a magnifying glass to examine them and the edge of the board. Wire solder joints should be shiney. If you see a solder joint that looks dull or frosted, it is suspect as being a "COLD" solder joint. I resolder them since the joint is prone to failure.

There is a simple "tap test" for the computer that is done when it is running good or during a fault occurrence. Simply take a slender screwdriver and tap all over the sides and ends of the computer with the handle. If the problem starts up or quits, the computer is suspect.

You may have to remove the tape from a harness to examine wires for cracked or melted insulation which would indicate a problem beneath it.

Note: Sensors or coil low voltage wires that have double nut connectors; the bottom nut must be checked for tightness. Remove the upper nut and wire and tighten the lower nut down. Hold it in place with a small wrench while you tighten the upper nut. DO NOT move the lower nut while tightening the upper nut.

Some connectors that have a little bare copper wire exposed that is CORRODED or DARK may need the connector replaced due to corrosion that has wicked its way into the bare wire. Battery cables are always overlooked for this condition. I have actually slit and peeled the insulation back on a positive battery cable that showed a small amount of blue corrosion and found corrosion extending six inches under the insulation. Any wire can have this occur to it.

Good luck and let us know what you find."
 
Back
Top