1984 3.2 stumbles and breaks down at 4,200 RPM

[hayday]

Ok..I posted here last Fall about this problem...I'm going to list what has been checked and changed out. First the history...

The car was not driven much for the 8 years prior to when I purchased it. When test driven, it stumbled for a sec at 4,200. When run up again, it went right by it so I figured it just needed to be cleaned up and tuned up. A few days later it would not go past 4,200.

So the car received a tune up, which included rotor, cap, plugs, plug wires, fuel filter. Also had oil change, new brake fluid, transmission fluid, stuff like that.

Since then, it has a new reference sensor, speed sensor, CHT sensor. The fuel pressure was checked and the readings were 40 psi at idle and 32 psi at 4,000 RPM. A new coil was swapped in and no difference. A new DME relay was swapped in and no difference. The strainer in the fuel tank was checked and found clean.

I was told that 4,200 RPM is above normal cruising RPM hence doubt there's a worn air flow meter due to worn spot. I don't know if it was actually checked. A comment was also made doubt it is Cat as it would have symptoms sooner than 4,200 RPM. The throttle switch was checked and was OK at wide open. I do not know if the injectors were pulled and checked for any kind of blockage.

Lastly, a local fella with a 1984 was kind enough to swap in his ECU. It has a Steve Wong chip in it. It did not solve the problem.

So.....that's what's been done so far. The specific symptoms are these. It does not cut right out at 4,200 RPM, it struggles. If you back off the gas just a smidgeon, it comes right back and runs great at 4,100 RPM. It you give it gas when it starts struggling, it gets worse, the tach drops like a rock and the engine makes small backfiring noises. Not loud, but a lot of them. This happens both under load and in neutral.

Any comments would be appreciated...

[ganun]

Try a borrowed AFM, if still bad I would change the f pump.

[hayday]

You'd change the fuel pump even tho the pressure is ok?

[ganun]

I thought about that after the post, you are right,......what does not support the FP in my mind is the fact that it does it with and without load on the engine therefore fuel volume delivery does not seem to be the issue.

[rusnak]

Here's an easy test for the AFM. All you need is a digital multimeter, a 9 volt battery, and a battery connector that you can get at any Radio Shack. Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

Have you had the coil checked? Have you checked all of your grounds, and the battery cables, etc? When the cap and rotor were changed, was the advance mechanism cleaned and lubricated? All working well? Was there any play in the rotor shaft?

[hayday]

Thanks Ganun

Hi rusnak...thanks for the AFM post. I'll try that...a new coil was tried and no change. According to the tech, all the grounds were checked. A new Interstate battery was installed last week. I'll ask about the advance mechanism and the rotor shaft... The car starts and runs beautifully until it hits 4,200 RPM.

[rick-l]

I've always wanted to tell someone to look at the positioning weights in the distributor and have that be the cause.

[tazzieman]

Originally Posted by rusnak

Here's an easy test for the AFM. All you need is a digital multimeter, a 9 volt battery, and a battery connector that you can get at any Radio Shack. Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

I did this fix (realigned potentiometer) on my 944 and it solved my stumbling at the same rev range. A bit fiddly , but saved quite a few $ and was a very satisfying free fix!

[hayday]

Thanks for all the replies...I'm writing them all down. It's going in for a valve adjustment next week and I'll give all these suggestions to the tech......

[Chuck.H]

Let's revisit this comment you made:

>>> It (sic) you give it gas when it starts struggling, it gets worse, the tach drops like a rock and the engine makes small backfiring noises. Not loud, but a lot of them. This happens both under load and in neutral.

The tach drops? When you are in gear and trying to accelerate? Big clue here if this is the case. When you are just cruising along, the tach can't make sudden movements, because the transmission will keep the engine spinning, even if say the fuel pump stops. If the tach drops like a rock when you are in gear and driving, then I'd strongly suspect the speed sensor.

Hope this helps,
Chuck.H
'89 TurboLookTarga, 358k miles

[hayday]

Hi Chuck...
Thanks for your comment.

Yes when it begins to struggle at 4,200 RPM, if I hit the gas the tach drops very quickly. I told the garage this and they said the speed sensor checked ok and was the right distance from the flywheel. They said they replaced it last Fall. The comment was also made that a speed sensor should cause problems all the way up to 4,200 RPM not at just one spot...

[Chuck.H]

Originally Posted by hayday

Hi Chuck...
Thanks for your comment.

Yes when it begins to struggle at 4,200 RPM, if I hit the gas the tach drops very quickly. I told the garage this and they said the speed sensor checked ok and was the right distance from the flywheel. They said they replaced it last Fall. The comment was also made that a speed sensor should cause problems all the way up to 4,200 RPM not at just one spot...

Ok, since the tach is controlled by the DME, there are not many things that can cause it to drop so fast. Some that come to mind:

1) speed sensor read error
2) DME relay failure, causing DME computer to lose power
3) DME computer problem, causing computer to stop functioning momentarialy.

When you get it solved, please let us know what the final problem was!

Chuck.H
'89 TurboLookTarga, 359k miles

[hayday]

I will let everyone know...

[hayday]

By the way, if it was the speed sensor, why would it lose power in neutral without the car moving?

[hayday]

Sorry about that one...I was thinking about the speedo not the tach...my bad...

[blucille]

don't overlook the obvious, just because it had a new cap, rotor, wires, plugs a short time ago doesn't mean they are right, or ok. In fact, I'd say it increases the odds someone screwed up...do an idiot check, pull all the plugs, make sure they are ok, look for cracks in the cap.

[hayday]

Problem is, I'm not sure I'm going to get it solved! Ben quite a while now.....

[frannyB]

You say the car sat for several years... Any chance you had little furry visitors set up home somewhere? Maybe munching on the wires a bit?

Can you check the sensors at the computer to be certain the actual wiring is good? Just a thought...

Did you replace the air filter? It might be due after all those years... that wouldn't be specific to 4.2k rpm though.

It does sound electrical though. Our 356 would falter over 2500rpm and it turned out to be the condenser. I know that is long gone on an '85, but it does sound as though your ignition power can't keep up. Or it is being rev-limited on purpose. Some rotors have that built in.

Just some thoughts...

Franny

[Chuck.H]

At this point I'd put an oscilloscope on the DME inputs (starting with the speed sensor :-)) and run it up - good thing is you say it happens in neutral, so you or your mechanic can do this in the shop.

Even if the speed sensor is good, as Franny says, critters could have chewed the wires.

I've found that you can remove the cover to the DME, pop the boards apart and put the test leads right on the connector wires on the DME board itself, which tests the sensor, wiring harness and DME connector all at once.

Chuck.H
'89 TurboLookTarga, 359k miles

[DRACO A5OG]

Hayday and all,

Next time if you suspect the DME, place the suspect DME into the known good car, never the other way around if the failing car has issues it may damage the good DME.

Have your tech check your Dizzy for slop or do it yourself. Place the car in TDC and remove the dizzy, it should spin with some resistance, a normal condition. check for play horizontally, if more than .5 MM your shims ( stainless steal washer ) are worn away and need replacing. If you have axial play the dizzy is shot, the internal snap in bushing are not available from BOSCH. Ask me how I know.

If you could place your AFM into your buddy's car, it would eliminate that as a suspect or do the 944 bench test. Just be very carefully how much tension you put on the arm as not to damage the poteniometer's film.