348GTS stumbling, backfiring and undriveable when hot...

I had a similar issue, and it turned out to be the crank sensors, but you said that is sorted out.

Any Check Engine Lights?

Cam sensors and throttle position sensors are also pretty easy to check with a multi-meter, but I don't think that is your issue.

Based on your description, I'll throw this wild guess out there: failing fuel pump or clogged inlet. There is a known issue with rubber seals in the tank affecting the pump inlets.

Reasoning:

1. Runs fine when cold, while ECU is running in extra enrichment mode, so more fuel is being pushed.

2. Runs OK with lower fuel maps.

3. Hesitates at higher RPMs when fuel demand increases and maps go into WOT mode. Increased fuel demand could cause sediment to push up into filter / inlet.

4. Stall at idle. Assume for the sake of discussion you are only putting out 75% fuel needed. At idle when warm, this would not be enough to hold a steady idle, but would likely be ok for cruising when the throttle is partially open, albeit a bit lean. ECU will correct for some fuel issues, but can't handle it all.

5. Intermittent issues. Pump flaky, or debris being sucked up and dropped.

Amazed no check engine lights, like a 1212 or a 1211?

ALL WILD SPECULATION but if I'm right you can send me a 6 pack.

 

Brian, Dave, Ernie, Plugzit or any of the other pros or enthusiasts can make better test recommendations than I, but I suspect a fuel pressure test would be telling.

There are many other things it could be as well, of course. The thing that prevents me from suggesting them is the lack of a check engine light. You are 100% sure they are working and installed? Have you tried pulling the codes just to make sure the lights and ECUs work ok?

You could pull the O2 sensor connectors off and force the ECUs into open loop mode. That should test your ECU CELs, and tell you if there is something in the emissions system causing a problem.

Other crazy ideas in the hopes it might spur some help. Not all of these match your symptoms, but I've been where you are, and I know how it feels to just want ideas:

Bad gas (simple stuff)
Timing problems
Bearing getting hot somewhere and seizing
Failing fuel pump (getting hot and pulling too much current, slowing down)
Bad cat (check the temp on both sides. Might have something loose and clogging one side)
Idle Air Control Valve stuck (don't think symptoms match)
Vacuum leak (gap somewhere that expands with heat)
Bad connection getting hot and backing out in wiring harness
Fuel pump relay getting hot and cutting out
ECU resetting mid drive
Wire grounding on firewall to an ECU
Voltage issues causing ECU reset with heat
Bad alternator losing output with heat
Stuck freaking parking brake!?!?!
Bad clutch problems putting too much load on motor?
Voodoo curse?


There are small air inlets on the intake, near the TB that are known to get clogged. Doubt this would cause your high RPM issues, but can cause idle problems.

Any way you slice it, it your car is stalling AND you are not throwing any codes, something is majorly messed up. Not many problems I can think of that severe which the ECU can't detect. This logically suggests that either your ECU or CEL system is not functioning, or it is something in a system that the ECU can't detect. But heck, stumbling and misfiring should trigger something.

My advice: Make sure your ECUs are throwing codes, and not resetting. Both sides. If not, you've narrowed down your problem domain a lot. If they look good, check that fuel pump relay. A brief interruption (relay losing contact) might be enough to screw your fuel pressure but not trip a CEL.

If none of these ideas help, I'd consult with a pro for a diagnostic-- and I would definitely not push the car. If you have a major mechanical issue, a pro's wisdom may save you a lot of money in the long run.

(P.s. Don't run it to 7800 RPM cold anymore, either. )

 

You are talking about kilometers. Do you have a euro version? If so, you don't have any Check Engine lights, but that's an easy fix. It's easy to pull the codes yourself.
BR
Bertil

 

i am gonna guess you have a fueling issue. Pumps, filters, or fuel pressure regulators.
Start at the easy spot the fuel pressure regulators.
Start the car, let it idle for 5 min or so. Shut down and pull the FPR vaccum hose off. Any fuel in there? If so you have a bad fuel pressure regulator.
My 348 has 1 bad regulator and i get back firing when slowing down or coasting, hard starting, and a terrible eye buring fuel smell at idle. I also failed an idle emissions test. New FPR on the way.
Rock auto has them for $80 a pop

 

Do these cars have knock sensors? Sounds like a knock sensor freaking out and the computer is pulling timing out. I'm not sure the computer can pull enough timing to stall the engine or at least it can't on a Volvo, of which I'm far more familiar...

I think the first problem could be completely unrelated. Sounds like a sticky idle air control valve to me.

 

Hi,

My 1990 348 recently had the exact same symptoms, except possibly the stumbling/hesitation at 7k rpm.... turned out to be one of the fuel pressure regulators. Pull the vacuum hose off of the FPR fitting and see if it has gas or a heavy gas smell in it - if so it's leaking. If you happen to have a small hand vacuum pump, you can put the hand pump rubber hose on the fitting, apply a small amount of vacuum and see if any liquid gass starts coming up the hose. There should be no gas in the vacuum hose at all. If there is, raw unmetered fuel is being dumped into the intake manifold, which the engine can withstand when the engine is cold but will cause it to run rich, stall on return to idle, etc.

I bought both of mine from Daniel at Ricambi, installed them, and noticed a giant increase in throttle response, no stalling when returning to idle.

Hope this helps,

Joe

 

Here you can read how to pull the codes: My Ferrari 348
You don't need the cable and push button, you can use a paper clip instead.
BR
Bertil

 

Here is a pic of the cable and push button. It's supposed to be connected to the small "rat tails" coming out from the ECU's. Instead of the cable and button, you can use a paper clip directly to the ECU cables. But first, as you have the euro version, you have to put in the bulbs in your cluster. It's a 10 minutes job.
Bertil
Image Unavailable, Please Login

 

Do you have spark? Do you have injector pulse? Why are you throwing parts at it, without checking the basics?

 

First, please fill out your profile. We like to know who we are talking to.

Okay now.

This can be a few things.

1) the idle air control valves need cleaning

2) the throttle blades are closing fully. This can be adjusted with the throttle stops on the bottom of the throttle bodies. Also, check to make sure the throttle linkage is adjusted to the same length, and that the tb plates are opening/closing both at the same time.

3) the air bypass screws on the sides of each throttle body need to be adjusted

4) both plenums need to be pressure balanced in addition to adjusting the air bypass screws

5) your fuel preside regulators could be going bad

6) if you disconnected the battery you MUST let the ECUs go though the initial start up calibration. The Motronic uses the initial resistance values taken from the throttle position sensors at rest, and then uses that info to determine the degree the tb's are open/close.

Here is what I would try.

1) disconnect the catalytic converter temperature control units from the main wiring harness. If one is going bad it will send a signal to the Motronic ecu to shut the back down. So to rule that out, disconnect the stupid things. I have been running my 348 for nearly 13 years with the damn things disconnected. The Motronic will store a silent code (4121) but the engine will run.

2) check your cam faze sensor on the rear of the 1/4 bank intake cam. Make sure it is in good shape, that all the connections/contacts look good, and that the seal is not leaking oil into the sensor housing

3) check the condition of the fuel pump relays the passenger footwell

4) check to make sure your catalytic convert is not clogged with broken down ceramic matrix

5) when was the last time the fuel injectors where flow tested and cleaned? It would be a good idea to send them out for testing and cleaning.

 

Glad to hear it's sorted out. Thanks for the update.

 

That must be a huge relief! How did you figure it out? Was the wire visibly damaged, or something more subtle?

 

Great news.