GrimshawK

My 86.5 928 will not stay running. It started on my way home two weeks ago when it stalled on the road while driving. I restarted it and it would idle but would not accept any throttle. When you apply any throttle it instantly stalls. Did not matter how much.

Since then I have;
- tested the fuel pump and it seems OK
- Replaced the fuel filter, cleaned the tank screen.
- Replaced the fuel pump relay,
- Replaced the ignition relay.
- Checked the throttle position switch.
- Removed a mouse nest from the air filter (??? MAF damage??)
- MAF looks good

On cold start engine runs perfect for about two to 5 minutes at any throttle position then starts to fail. If i apply any throttle it will instantly stall. It will eventually die all together but generally will restart and only idle.

I feel like I am loosing my old friend . Any thoughts would be appreciated.

Kirk Grimshaw
86.5 928s

Speedtoys

So, is looking at the MAF a good test?

Mrmerlin

how many miles on it?
did you ever do an intake refresh?
the ICV is a wear item.
NOTE did you replace the TPS short harness yet? they all break
the temp 2 sensor should also be swapped out and the connector replaced.
Has a CPS ever been replaced?

dr bob

Was there any backfire or other indication of system upset when the car initially failed?

The test protocol looks eerily familiar...

-- Check for good spark

-- Check for obvious vacuum leaks (hoses popped off, manifold connections, etc.)

-- Check for fuel pressure. Use a gauge.

-- Borrow a good MAF. As Jeff says above in so many words, the symptoms say MAF is on the troubleshooting list. Unless the sensing wire has actually broken and is dangling obviously in the little nozzle, there is no visual check that will tell you whether the MAF is working correctly.

-- What did you work on last, prior to the symptoms showing up?

-- Did you very recently fill the tank or do anything beyond "working on" the car prior to the symptoms showing up?

GrimshawK

97,000 Miles

I did an intake refresh 2 years ago with a new MAF.
I did not replace the TPS harness yet. I tested it at the harness to harness connection and read proper closed and fully open throttle readings.
I have not replaced the IVC yet.
Good idea on the temp 2 sensor.
I replaced the CPS 5 years and about 25k miles ago.

What would be the specific failings of the Temp 2?
Is there a IVC test?

GrimshawK

Sorry dr bob I missed your questions...

There was and is a backfire when it starts to fail. It runs for about 4 minutes from cold with good throttle response then it will backfire and stall if any throttle is applied. I have tried running it for this 4 minute period at about 2000 rpm and it will run for the period, backfire and stall. If I let up on the throttle at the backfire it will settle down and idle smoothly but if you touch the throttle it stalls. The stall sounds more like a shut off than a falter.

Spark is good cold I have not yet tested when it fails.

I assumed speedtoys was just kidding when he mentioned the validity of the visual MAF test. It has not been tested yet and I do not have a spare. I was hoping to work through less invasive tests first.

The car has not had any major work done to it in the last two years. Very few miles driven though. I had just finished a 150 mile drive. Parked it for about 2 hours and continued my drive home. It ran 15 minutes then died a half a mile from home. The only prior symptom has been that at times it will not start consistently meaning that I have to cycle the key multiple times to get it to fire. It will crank but not fire. Cycling the key/switch will generally engage what ever relay or contact that is hanging up. I have replaced both the fuel pump relay and the fuel injection relay.

Thanks to all for your input so far!

dr bob

There's a "Dwayne" guide to intake refresh that includes a diagram of some plumbing pieces that will let you look at intake leaks. You have some classic symptoms, especially considering the backfire(s) you've experienced. It Just Takes One to pop a connection loose, and from then on you'll have the symptoms you describe.

I don't have a map handy that shows where Montague, Mi might be. Unless you are isolated in the UP somewhere, you are likely within handy driving distance of another LH 928, on which you can test your MAF. Run the flag up and see who salutes.

Double-check the connections at the MAF. Only once (so far) I had some very interesting driving symptoms that turned out to be some connector issues. I had narrowed down some of the possible causes as I nursed the car the last mile or so to the garage, but still I needed to actually pull the connector and look to see what was needed. Easy enough to fix once the problem was identified.

The temp-II sensor adjusts the mixture as the engine warms up. Your symptoms point to having adequate fuel while cold, falls off as soon as you move off of cold. Checking the Temp-II sensor is pretty easy with a DMM, and there's a target-values table in the workshop manuals that will tell you what you want to see. If you powdercoated the coolant crossover, make sure that it's grounded well now, and also make sure the threaded hole that the temp sensors use are free of paint. Good Idea to clean the connectors and use a little DeOxit on that and the sensor pins just to be sure. Sensor is cheap enough to replace, but that won't solve poor ground path issues.

Your list of things checked in your first post is good. Going to the fuel pump comment that it "looks good", confirm that it's actually delivering correct fuel by connecting a fuel pressure gauge at the front rail cap.

You can check for spark easily and non-invasively with a common clamp-on timing light. It won't flash is there's no spark. Coils very infrequently fail, and when they do it's rare that they work when cold, fail when warm. Still worth a look if you find that you lose spark when warm. Ditto the ignition "amplifier" module(s), although they are heat-sensitive and subject to water exposure. Caps crack and suffer from carbon-tracking, ignition (coil) wires come loose, both worth inspection.

Is the tach bouncing when you crank after the engine fails to run? You had the CPS connector out, hopefully put a new one in with your intake refresh effort. Connector still OK and seated well? They are notorious for failing with heat/age, relatively painless to disassemble and inspect.

At all circles around 2 of the three the basic needs: fuel and spark. (other is compression)

mj1pate

I just want to mention that all of our ~30 year old cars need broad scope electrical refresh. ECU capacitors/solder joints, harness ring terminal/multiple wire connectors, temp sensors, worn insulation in the engine compartment, etc. As you get into detailed inspection, all these things show up. Just be careful. Change one (type of) thing and check results/operation before changing anything else.

dr bob

More thoughts:

Verify that the MAF is fully seated in the boot to the throttle assembly, and that the clamps are fitted correctly.

The oil vent hoses are a critical part of the intake -- leaks in those are the same as air leaks into the intake. Worth inspecting.

Is the oil filler cap fitted correctly?

The hose(s) to the idle air controller need to be fitted perfectly, and are known to pop off and cause air leaks into the intake when not fitted well. An inspection camera snaked under the front of the intake might help identify this. The intake air leak tests will give you some guidance on this and the oil vent and filler symptoms.

The fuel pump receives power via a dedicated small positive cable directly from the battery positive terminal. Make sure all the connections at the battery positive are clean and tight, including those smaller wires. (others are LH and EZF.) Know that the fuel pump relay also powers the heater in the oxygen sensor. If that oxy sensor heater circuit is shorted to ground, it will cause low voltage at the fuel pump.

Does your car have the in-tank fuel pump? Telltale is wires that pass through the fitting at thge fuel tank discharge nozzle. If that pump fails, flow is significantly limited.

Both of these fuel pump symptoms will show up when you test fuel pressure and flow.

---

Stuttering and backfire, especially while cranking, can upset the timing belt tension enough to cause it to jump a tooth or two. Worth a look by pulling the two cam gear covers and lining up all the timing marks.