Intermittent stalling/ missing and starting

desertt5 · 03-31-2010, 09:45 AM

My 944S is acting like a little beotch. Intermittently ( once every 40 miles) it will cut out while driving. You can feel it jerk, but always comes back. Has actually slammed my wife's head of the head rest when it did it in first gear on her. To me it seems like it is losing fuel, but maybe not. It has done it cold and after a 20 mile drive, and from 2000 rpm up to 3500 rpm.

Within the last 3 weeks it has started, made it to 500 RPM then died. After 2 - 5 minutes it started and ran fine without me dong anything, except waiting impatiently and cussing.

So far I have a new DME relay, fuel filter, fuel pump. The FPR and dampener are less than a year old. Took out the DME and checked solder joints and cleaned some minor corrosion. ( This was actually by our solder pro's at work).

What to check next?

Also, where does the tach gets its signal from? After I put the DME back in, its been dead. Did I miss a connector down there? I was trying to get it back in in the rain so I could go home. Won't be able to pull it again till Friday to look.

SolReaver · 03-31-2010, 10:06 AM

OK, fuel filter and the fuel screen at the gas tank. Battery connections and Grounds (yes, ALL of em). Cap and Rotor. Sensor connections, gap, and the sensors themselves. Pull the coil boot and check the coil connections and the coil. Clean the fusebox fuse connections (it can't hurt) clean ALL the sensor connectors and (carefully) check the ECM connector and Ground to ECM

As you know, the problem is intermittent and hard to get at because it works most of the time. I am leaning towards electrical or fuel. It would help if there were any times that it always misbehaved, like when damp, low on gas, etc. Otherwise, we just have to go after the usual suspects until the problem goes away. If the main power stays on you can concentrate on the power and grounds for the running circuits. You MIGHT want to hold off on pulling the sensors just now as they are a pain at best. Just check their impedance at the connector.

Aenyone · Aenyone's Garage

Are we fighting the same issue?

Lunging/Loss of Power/Nosedive

SolReaver · 03-31-2010, 03:48 PM

Yea, I was noticing some similarities there. Same goes for you. go through the simple stuff first. It MIGHT turn out to be a FI component, but you want to cover your bases. Feel free to bounce ideas off each other and how you ruled out components as the source of the intermittent failure. With FI it is usually bad connections, sensors, or faulty grounds, but, electronics DO go bad.

desertt5 · 03-31-2010, 07:11 PM

Thanks. I cleaned all the grounds a while back ( seven of them?) and hit them with clear silicon, but will look at them again. ECM to ground huh?? Ain't touched that one yet.

Will check cap and plug wires this weekend. Intermitent coil?

If only one injector was cutting out, would it be noticeable at 70 mph in 5th?

It feels like it would have to be ALL my spark I'm losing. Could you lose fuel pressure that quick and gain it back instantly? It can feel like 3 quick jerks at highway speeds.

( just thinking out loud again)

Now where did I leave my beer..........

desertt5 · 03-31-2010, 07:40 PM

Quote:

Originally Posted by Aenyone

Are we fighting the same issue?

Lunging/Loss of Power/Nosedive

They do sound similar. Mine happens really quick and then goes back to normal.

Where are you located?

SolReaver · 03-31-2010, 08:06 PM

The ECM is grounded via it's case as it is bolted to the frame. It also has grounds that go to the pins. I was speaking of the case grounding to the frame, and of course, the frame grounding to the battery. Basically, just check that the ecm case is solidly and cleanly bolted in. Make sure the big cable is snug by removing it and putting it back on while you remain grounded (disconnect battery before doing this stuff..OK?)

By your description, I was thinking that there was (or is) some kind of shutdown and the engine was rapidly decellerating the car. I have had 2 cylinders go intermittently (bad day) and it is not enough to lurch you forward. It is rather noticeable though. I have also had 1 injector go bad intermittently. again, noticeable, but not "who slammed on the brakes"?

Coils usually work or they don't. Now, power or signal TO the coil, yea, that's possible and so checking the connections is warranted.

"Quick jerks" huh? Is it like bad gas? or water in the gas? Tried a can of gas line anti freeze? If you have a fuel pressure gage, you could drive around while watching the pressure and then know if it is a fuel prob. I am going on the belief that you have good voltage to the FPump and that the relay is good. You could rig a test light to the FP power and again, drive the vehicle to test that one as well. You could even rig a test light to the coil power and see if the ignition shuts off.

Could you loose FP and then have it snap back? Well....with a gage on the rail I can tell you the pressure comes up pretty fast. On the order of a half second to a second (depending on the FP, temp, and voltage) so, yea, that IS possible.

Does it only misbehave under load? Say a good hill climb or highway speeds? that, to me would indicate a fuel starvation condition which could be anything from a clogged filter, to a bad FP, with a FP regulator in between. Can you MAKE it misbehave? if so...How?

Speaking of fuel starvation...I need to get me a beer too...

Aenyone · Aenyone's Garage

Quote:

Originally Posted by desertt5

They do sound similar. Mine happens really quick and then goes back to normal.

Where are you located?

Over in Lafayette, Louisiana.
I'm going pick up a buddy's AFM to swap. I'll post the results on my thread and let you know it if helped.

944 Ecology · 04-01-2010, 06:59 AM

Quote:

Originally Posted by SolReaver

The ECM is grounded via it's case as it is bolted to the frame.

Unfortunately, that is not the way it works. Any (trust me I know) 944 (S/S2/Turbo, too) DME will work perfectly without the case being grounded.

SolReaver · 04-01-2010, 04:22 PM

REally? Well, that is odd....I have fixed a few bugs this way. Perhaps it is just the re seating of the cable that made the difference. I guess the additional ground I installed from the frame to the ecm is superfluous. Oh well, I will keep it as I am sentimental and superstitious. Thanks for the correction / clarification. and, oh yea, I trust you on this one. Any wild guesses as to where this gremlin lies? the similarly afflicted owner is thinking AFM.

P944GUY · P944GUY's Garage

mine was doing the same thing a few years back and it was the computer..would just quit running while driving..did it sporadically and then just finally died....

desertt5 · 04-03-2010, 10:27 AM

Today while backing into the drive, it barely had enough power to pull itself up the small slope. It sounded like it was hitting on 2 or cylinders. Now its back to runs for 1 secthen dies. It is definately a fuel problem. I unplugged the fuel pump and it still does the same thing.

I have 12 volts at the fuel pump with the DME relay jumped. Going to look how to check power at the injectors. The FPR and dampener are about a year old only.

desertt5 · 04-04-2010, 06:49 AM

Found a good DME. It should be here soon and we will see.

SolReaver · 04-04-2010, 12:24 PM

Finjectore signal is best tested with a "noid" It would help to know pressure at the rail. Did you set up a simple test light in circuit? I understand the wanting to replace the DME, or at least test it. However, understand that you may be running a small risk of damaging the known good one. Could you just pull the rail and check to see the injectors are firing? Have you checked for spark on all cylinders? have you disconnected 1 spark plug wire at a time and found a difference? (no dif, FI not firing)

I just don't like randomly replacing parts that might be bad. I prefer a more logical and reasoned approach with a side helping of gut instinct.

desertt5 · 04-08-2010, 05:30 PM

DME got here today, thanks running_cold924. Put it in and fired right up. Just to see I put the old one back in. It did start after 3 tries, but ran like it was hitting on only 2 or 3 cylinders.

Put new DME back in and she fired right up smooth.

Will know tomorrow when I drive 40 miles round trip to work.

P944guy, your post is a big reason I said screw it and got the DME.

thanks for all the replies

Aenyone · Aenyone's Garage

I'm glad you got your problem fixed!
I had already swapped a DME in mine, and no change.....

I'm changing the fuel filter tomorrow, due to the results of the fuel pressure test.

M758 · 04-09-2010, 07:34 AM

I have been chasing a somewhat similiar issue in my 944 spec car. The runs fine and is fast then installed the tach drops and engine cuts out. Then comes right back 1/2 second later.

At idle it runs fine then cuts out in a similar way then comes back. If the RPM of engine is low, like at idle, it can stall. I never stalls at speeds since well... the engien never stops turning over.

At idle I can hear the DME relay clicking over right when it happens. It is electrical related and changing DME, AFM, Speed & Ref sensors, etc do nothing. I believe the probelme is bad wirring. I think is some wire damage under the coverings that I can't see very well. These are causing random shorts in the signals causing the DME to turn off. Then it comes back.

The solution is probably to rewire the DME harness. This mean wires for injectors, AFM, block temp sensor, speed & ref sensor, TPS, etc. I have jiggled all of these at idle and not found the issut, but it is still there.

My harness is old and brittle and the probelm cropped when I swap the enigne. Naturally I move the harness back in the process. So I guess it damaged the harness. Eariler this year I had the problem get very bad. It was then I notice the DME block temp sensor wires a little damaged. I patched them up to prevent shorting and the car ran better. Not perfect, but better.

SolReaver · 04-09-2010, 08:20 AM

M758: The FI connections in the harness you talk about are crimp connections and over time they do corrode. I had to split the harness almost to the firewall to get at all of them but about 8 inches short of that gets most of them (YMMV) If you manipulate the harness, they tend to let go. If you are missing cylinders due to no or low signal opening the harness is the way to go. Not surprisingly, there is little room to work with when the engine is in the car and there is a REALLY big clamp that holds the harness to the block in the dark area behind the engine. Leaking rad fluid from the hoses to the heater core can make the corrosion even worse back there. The wires are usually good, it is the connectors that rot away and so you may be able to rebuild the current harness in place as I did. The only way to tell is to carefully open the big snake and inspect. The shorts present randomly. IOW, they MAY OHM out well with a meter, but still fail in service. If they don't ohm out to the ECM connector, they are def gone. The circuit is depicted as paired with the ecm out, but test as one with the ECM connected as the ECM fires all the injectors at once.

SolReaver · 04-09-2010, 08:23 AM

Desert5: Way to go! ECM diagnosis is kinda a dark art as they present so many ways, Usually it is better to rule out the more usual suspects like connections and sensors. However, you got it spot on! Congratulations...

M758 · 04-09-2010, 11:54 AM

Sol,
Yeah I know those wires can get nasty. So far the problem is manageable. Routine stumble on idle. Maybe once a twice a day it will stumble on track. Funny though it ran pefect in downpour tons of rain.

No heater in the car so I have no issues with water in the fire wall area. One day I will get to the wires.