engine stuttering

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Rettax3
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Re: engine stuttering

Postby Rettax3 » Sun May 26, 2013 10:31 pm

It is likely that your engine's problem is intermittent, maybe one cylinder isn't hitting every time, like it is losing spark sometimes or not firing correctly because the fuel mixture is too far rich or too far lean. At low RPMs, it would be much more noticeable, because the time the engine can recover from this power-loss is longer until the next cylinder can catch it back up. At high RPMs, a few lost power-strokes every few dozen revolutions or so would be easily missed.
Also, at idle, your engine is making just enough power to turn itself, so a little loss is really noticeable, whereas at high RPMs the engine has greater power available to it, and most importantly it has more inertia trying to keep it turning. Think of it as trying to roll a tire up over a bump, if it is moving fast it goes over it easily, if the tire is barely rolling, the bump can stop it cold.

When you are looking at the plug wires, check them at night and look for spark, but also go over the whole length of the wire and look for cracks, splits, or discoloration, like brown/yellow spots, black areas, or an off-white sugary powder on them -this is carbonizing caused by the high voltage spark arcing to ground.

The Beretta community is really one of the most supportive and also most entertaining I've found. I don't much bother with the sites for the other cars I own. I think the Fiero sites might be the only other ones even close, some of them really are pretty good too.


1989 Super Charged 3800 Series II (First)Six-Speed Conversion GTU
1990 Turbo 3.4 Five-Speed T-Type
1990 4.0L 4-Cam 32-Valve V-8 Five-Speed Indy GTi (Rebuild Project)
1990 Stock(!) 3.1 MPFI Auto Indy
1995 3400 Top-End Auto OD Z-26

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3X00-Modified
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Re: engine stuttering

Postby 3X00-Modified » Thu May 30, 2013 9:01 am

If you haven't swapped it out yet, ICM could be a problem, or a coil, they are stuck under the exhaust manifolds and see a lot of heat and can cause a intermittent miss.

Injectors also have a tendency to go bad on these engines. Unplug the main injector harness behind the plenum and do an Ohm check on each bank. I don't know which pins are for each bank but try to see wire colors and trace it, see if the banks match, each injector should be between 12 and 13 ohms, so it will be around 4-5 total for the bank since they are wired in Parallel.



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Berry
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Re: engine stuttering

Postby Berry » Sat Jun 01, 2013 6:59 am

do I have to open the upper intake to check the resistance of the injectors?
I checked the ignition wires in the dark with waterspray. no sparks no flashes. I will check their resistance soon.

maybe the valve in cyl no 5. has to be adjusted. that would also explain the oil fouled spark and the oil in the intake? what do you think?
or troubles with the MAP sensor ?


...cruising
92 GT 3.1 MPFI auto 57K white

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Rettax3
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Re: engine stuttering

Postby Rettax3 » Sat Jun 01, 2013 3:43 pm

You should be able to get to the 4-pin injector wiring harness toward the back of the intake manifold without removing the upper intake. The wires are light green, light blue (these are the two wires that go to the computer, and are grounded inside the computer in pulses to fire the injectors) and two pink wires (these are battery + voltage with the ignition on) -I do not recall if any of them have black stripes on them too, I think both pink ones are striped. Check resistance from the pink wires to the green wire, then check again the pink wires to the blue wire.

Still inspect the ignition wires visually for cracks, discoloration, off-white powder. If they are bleeding-down to ground in a place where they are physically touching the engine, you may not see sparks.

You cannot 'adjust' either valve for cylinder #5. If you have to replace an oil-seal on the valve-stem, you probably have to remove the cylinder-head to do it. :( At that point, it might be worth while to re-hone and re-ring the cylinder too, since shipping parts is so expensive.

A bad MAP sensor or a leaking vacuum-line to the MAP sensor could be causing your trouble too, it is worth looking at. And a spare MAP sensor in the glove-box isn't a bad thing to have, so even if it doesn't help, it isn't a total waste.

Good luck.


1989 Super Charged 3800 Series II (First)Six-Speed Conversion GTU
1990 Turbo 3.4 Five-Speed T-Type
1990 4.0L 4-Cam 32-Valve V-8 Five-Speed Indy GTi (Rebuild Project)
1990 Stock(!) 3.1 MPFI Auto Indy
1995 3400 Top-End Auto OD Z-26

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Berry
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Re: engine stuttering

Postby Berry » Fri Jul 17, 2015 4:00 pm

problem solved! just for the information to all others who have the same issues with the idle: i changed the ECM and everything runs fine.


...cruising
92 GT 3.1 MPFI auto 57K white

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Money pit Beretta
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Re: engine stuttering

Postby Money pit Beretta » Fri Jul 17, 2015 7:16 pm

That is great news! sorry to lead you down the wrong path.


keep'em flying!

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Berry
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Re: engine stuttering

Postby Berry » Sat Jul 18, 2015 8:51 am

NP money pit beretta :D
many thanks to all for your efforts!


...cruising
92 GT 3.1 MPFI auto 57K white

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