View Full Version : Gas choking out when hot

06-02-2008, 03:46 PM
"Hi guys ,am I glad to find this site. I have a 1996 5.0Fi volvo penta with cobra sx nice motor ,runs great,almost . The problem is ..the motor starts right up idles well ,good oil pres, good volts from both batts.,temp is right at 155-160 always, after it runs for a while ,1/2 hour approx. and at idle neutral or ide speed the motor starts to choke out,if in idle speed and hit the throttle,she backfires through the flame arrester,with a surge bog down,surge bog down , very frustating ,I have not gotten any good ,staight answers to this problem is there a fix for this ,I hope ...craig"

06-03-2008, 07:17 PM
"Have you checked your timing? Even if fuel injected, it could be off.
Also, being fuel injected means that it is computer controlled. So there is a way to check it to see what the computer is seeing when it "acts up". good luck....."

06-04-2008, 12:57 PM

I am not overly familiar with VP's fuel injected motors, but I have a good amount of experience with FI in general. I will offer some general advice:

Most marine FI systems operate on an OPEN loop system, meaning the ECM (electronic control module) does not trim the fuel delivery based on combustion byproduct (commonly done with a pair of O2 sensors in automotive apps.) However, when the engine is cold, the ECM calls for more fuel to be delivered (static predetermined values) until the ECS (engine coolant sensor) reports that the operating temperature has been reached. Once at operating temperature less fuel is delivered. This is very similiar to the use of an electronic, or thermo spring choke on a carbed engine.

Your setup; and I am assuming you have a 5.0Fi PMDA configuration, utilizes a few sensors to provide input to the ECM.

When the engine is cold, does it run OK? This could signify a problem with an ECS input. (Perhaps unlikely since it is holding consistent temp.)

The next sensor to investigate is the MAP (manifold absolute pressure) sensor. Engine RPM, air temp. and the MAP sensor provide data to the ECM to calculate the amount of fuel flow required based on a speed-density calculation. Remove the sensor (part #75) and inspect the ports. Is there carbon buildup? You can clean a MAP sensor with electronics cleaner (DO NOT USE BRAKE CLEANER).

Also, verify that all vacuum lines are in place and not cracked. This is very important since vacuum leaks adversely affect MAP sensor output.

Check the PCV valve and hose as well (part #1 & #57).

Next I would look at the throttle air bypass (part #26). This device allows 'bleed' air into the intake during warmup and idle. Many configurations have a solenoid that modulates airflow. (Automotive engines use an IAC - idle air control, valve.) See if there is a solenoid on your setup and ensure the wiring is secure and the solenoid is operating.

There should be a TPS (throttle position sensor) on your TB (throttle body). Take a look at part #19. This sensor tells the ECM the position of the butterfly in the TB, thereby increasing or decreasing the fuel delivery. VP lists the part as 'SENDER'. Verify the connection, etc.

From here I would look at the TFI (thick film ignition) module. The TFI and ECM control the spark output of the ignition module. Ford cars had problems with TFIs overheating many years ago. This lead to intermittent stalling and lose of power under load. Aside from removing, inspecting and cleaning, there is very little you can do - aside from replacing with new. (Part #62)

Also check out the fuel filter (part #38) and ensure that it is clear.

Pull some plugs - report how they look, is the engine running lean, or rich?

Often times problems with FI systems tend to be mechanical - mostly vacuum leaks and not electronic issues.

Check the manifold gasket (part #70) to see if you have a leak. You can do this by starting the engine, letting it warm, then with it idliing in neutral spray WD40 around the base of the intake. If the RPMs of the engine fluctuate there air (and WD40) is entering.

Sometimes it is less costly to replace a few sensors and get the engine back in shape than it is to bring to the boatyard and get whacked with their diagnostic charge. If you know someone with a ScanTool, that would be the best choice.

Let us know how you make out, good luck.

Schematic: http://www.volvopentastore.com/UPPER_INTAKE_MANIFOLD_5_0_LITRE_MODELS/dm/cart_id .327823918--category_id.342368--search_type.category--session_id.362536706--stor e_id.366--view_id.311975 ("")"

06-04-2008, 03:31 PM
"Mike, what a world of good information,it will take a couple of days for me to get through all this ,but I can tell you it feels good to get started on a very frustating problem,I will let you know., but first inanswer to your question, yes it starts, runs great when cold,or at cruising speed or above ,trouble is only at warm neutral idle, idle speedor idle speed to plane under load ,as pulling tube-wakeboard etc.,stall surge ,stall,surge,and then catches and takes off like a raped ape it will slowly cut out when sitting at idle or idle speed ,if not put in neutral and given throttle forward to about 1000 to 1200 rpm to clear out let me know ...Craig"

06-04-2008, 03:37 PM

You may want to start your search with the air bypass located on the throttle body.

Once the engine warms to operating temp the solenoid should retract (or extend, depending on the configuration).

If you can take a pic of the solenoid / TB and upload it for me to check out, I can probably be more helpful. Good luck."