1987 911 3.2 stumbling when hot

[DLK]

Hi all,

I have an odd problem with a 911 3.2, maybe someone can give me an insight as to what I've missed.

- car is originally from Canada and is in Europe now.
- when I bought it, it had a MAF conversion kit and a ridiculous exhaust.
- changed the MAF back to the original air flow meter and airbox, fitted a standaard (NO CAT) exhaust.
- the ecu has a chip especially for the MAF kit and changed the ecu with a ROW ecu.
- ROW cars do not have an O2 sensor and I disconnected the one on the car. No place to put it as it has a ROW pre muffler without O2 bung.

The issue

The car starts up fine and idles normally. When you drive off the engine is smooth and everything feels fine. Once the engine heats up just a bit the problem starts. When driving a constant speed it's bucking and cutting out. Feels quite random.

When you give it the beans and go full throttle it takes off smoothly. NO problems when you drive WOT.

What I've done and what I've found;

- exchanged the air flow meter with one from a car that runs fine --> no change
- exchanged the DME relay with one from a car that runs fine --> no change
- removed the sparkplugs, checked the gap, tightened the connectors on top of the plugs --> no change
- connection to the fuelpump leaked a tiny bit. Removed the fuel and it smelled old. Discarded the old fuel and put 20 liters of fresh premium gas in. --> No change
- I did NOT change the fuel filter yet!!
- removed the injectors, ultrasoon cleaned them --> No change
- sprayed brake cleaner on the gaskets of the intake manifold --> No change
- Plugged the old ECU back in --> No change

- on start-up it seems that cilinders 4 and 5 take longer to heat up (IR thermo-gun on the exhaust runners) and seem to stay colder
- car has the classic retrofit Fuse panel and the fuel pump starts as soon as you turn on the ignition. That's not correct!
- in the past someone ran a wire between the +terminal of the coil to the idle control valve. When I use the wire that runs to the idle control valve you can hear it humming with ignition off. So there is some issue with wiring, can't figure that out yet!
- car has an AGM battery, could that battery cause voltage spikes?!
- cap and rotor are older, look fine
- plug wires look fine and recent
- microswitch to indicate no throttle works
- microswitch to indicate WOT works
- when I unplug the head temp sender it increases idle speed

Other than changing the fuel filter and maybe put more fresh fuel in, im out of idea's. I can't imagine that the filter is the issue, on WOT it need the most fuel and it runs fine at WOT.

What is the next thing you would check?

[rowingone]

In tank fuel filter? Have you checked the gas tank for sediment? You could have sediment in the tank that momentarily blocks gas flow.

[DLK]

Originally Posted by rowingone

In tank fuel filter? Have you checked the gas tank for sediment? You could have sediment in the tank that momentarily blocks gas flow.

Good argument. But pulled the feed hose from the tank to the pump. Flowed freely

[911obgyn]

How old are the speed and reference sensors?
If they have been messed with perhaps gap?
You do have to get IAC valve wired correctly.

[proporsche]

all this work..I`d start with basics..that is --fuel filter-

Ivan

[RobFrost]

Check whether your fuel pressure control and regulator are leaking fuel into their vacuum hoses. Please let me know how you get on.

Also I think you'd gain some insight with an AFR sensor when running normally vs when the problem occurs.

Sent from my SM-G988B using Tapatalk

[yelcab1]

It seems you have a fuel problem. However, I would encourage you to buy a Wide Band O2 sensor (AKA Air / Fuel Meter) and plug it into your now un-used O2 sensor bung and see what the fuel ratio is when it acts up. That gives you a big piece of information. I once had a problem like this and could not solve it until I got that AFR meter.

I also think changing out the fuel filter and checking fuel delivery specifications is the next thing to focus on.

It would also be good if you can sort out the Idle Valve controller thing.

[rowingone]

The sediment in my tank caused an intermittent problem . . I would not have thought that . . but the gas would randomly stop flowing and then restart for no reason. Once I had it idling in the garage and the car just stopped running . . or once returning from a drive it stopped running in the driveway and wouldn't restart until the next morning. I believe the sediment right around the in tank filter would swirl around that filter and occasionally block it.

[Daves911L]

Your problem sounds very similar to what i experienced with our '88. Car was completely original and always well maintained. At around 90k miles it developed a habit of "cutting out" and "bucking" as it tried to light up again. This always happened while on the highway at steady speed and light throttle, and only when up to normal operating temps. Never when cold, or under acceleration, and it was intermittent. Some days or weeks it would not be present, but then would suddenly start up again. Sometimes it would happen once or twice, then stop. Other times it made the car almost undriveable.

Diagnostics consistently showed everything in spec. I changed a lot of stuff out of optimism. Fuel pump, filters, various components, pretty much everything that handled fuel. Ignition parts. Sensors, CHT, etc. Had the motronic checked for solder cracks and upgraded. Still it would happen. Finally I replaced the O2 sensor, even though it consistently checked out fine. It was effectively the only thing left. That cured it. 7 years and 40k miles later the probem has never recurred.

I believe the heating element in the O2 sesnor was the trouble. Not sure exactly how or why but I think the mixture was going way out of spec when the exhaust temp dropped and the O2 sensor was too cool for the engine temp.

[DLK]

Found the problem and it's my own fault for not thinking it through.

The DME that was in the car was mapped for use with a MAF-kit and a friend of mine has a project car and wanted to fit the MAF-kit to that car. So we swapped DME's

Turns out that the DME he gave me also has an altered chip to run with a MAF-kit. Engine was taken out of that car years back and we both forgot it had a MAF-kit fitted.

This morning I swapped the DME with the one from my 1987 Targa and it ran great.

All I need now is to find a standard chip (24-pin) or ask my local Motronic guru to program one for me.

Ps. came to the conclusion because I figured that when the engine is cold the DME gives it a bit more fuel (choke function) and it ran fine. Also in WOT it ran fine, the DME ditches the normal map and gives it more fuel.

[Cairo94507]

Thanks so much for circling back to let us know the solution- we all learn that way. Glad it is sorted.

[mysocal911]

Originally Posted by DLK

Found the problem and it's my own fault for not thinking it through.

The DME that was in the car was mapped for use with a MAF-kit and a friend of mine has a project car and wanted to fit the MAF-kit to that car. So we swapped DME's

Turns out that the DME he gave me also has an altered chip to run with a MAF-kit. Engine was taken out of that car years back and we both forgot it had a MAF-kit fitted.

This morning I swapped the DME with the one from my 1987 Targa and it ran great.

All I need now is to find a standard chip (24-pin) or ask my local Motronic guru to program one for me.

Ps. came to the conclusion because I figured that when the engine is cold the DME gives it a bit more fuel (choke function) and it ran fine. Also in WOT it ran fine, the DME ditches the normal map and gives it more fuel.

A waste of money & a total piece of junk!

[scarceller]

Originally Posted by DLK

Found the problem and it's my own fault for not thinking it through.

The DME that was in the car was mapped for use with a MAF-kit and a friend of mine has a project car and wanted to fit the MAF-kit to that car. So we swapped DME's

Turns out that the DME he gave me also has an altered chip to run with a MAF-kit. Engine was taken out of that car years back and we both forgot it had a MAF-kit fitted.

This morning I swapped the DME with the one from my 1987 Targa and it ran great.

All I need now is to find a standard chip (24-pin) or ask my local Motronic guru to program one for me.

Ps. came to the conclusion because I figured that when the engine is cold the DME gives it a bit more fuel (choke function) and it ran fine. Also in WOT it ran fine, the DME ditches the normal map and gives it more fuel.

Just curious, what MAF kit was it running? Not all MAF kits work properly.

Also, invest in a decent wide band o2 gauge, best $200 you can spend on these cars. You'll know exactly what the AFR is and you can then fine tune the base idle mixture using the gauge. At full warm idle the AFR needs to be 13.8 to 14.4 AFR with O2 sensor disconnect.

Enjoy.

[eisenbud]

Hook up a volt meter and measure the charging voltage while the car is acting up, we have seen these symptoms created by over charging. Also never spray brake cleaner near the intake of a running engine.