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Stumbling & no power when warm

3K views 24 replies 5 participants last post by  Del 
#1 ·
Hi guys,
A new issue has popped up on the 91 164S.
The engine is quite strong until it reaches operating temp. I have replaced the timing belt, fuel pump, filter, injectors serviced etc.
I have tried the self-diagnostic with the steering column and for some reason, it does not work, no flashes on the check engine light. Also, checked all of the standard stuff, no air leaks, MAF boot good, new plugs, wires cap, rotor. Where have I failed?
Thanks in advance
Al.
 
#3 ·
Check G131 grounds on end of the intake plenum near upper dogbone engine mount AND if later 164 engine check ground for intake plenum to rear valve cover if intake plenum setting on rubber mounts (extra grounding strap from throttle body to rear of head or valve cover bolt).
 
#7 ·
It manages it more when cold than hot. Engine when warm/hot will run just fine with this sensor disconnected. Try it and see if that cures your problem. It is a cold start device to enrich system with more fuel injection flow/pulse width when cold.
 
#9 ·
While you are at it, go through and clean and protect every electrical connection in the engine bay, regardless of how small, etc. Very easy chore to do, and ensures they are not a problem, as they can be in older cars. Old dirty connections can sometimes develop excess resistance if they get hot. My mechanic said once that Alfa paid them $5 per connection to do exactly this, as they sometimes were a problem in, at least, 164s.
 
#10 ·
Check and see if air flow meter spring loaded flapper vane binds when fully open or not going fully open without binding.

Check RPM/Timing sensor air gap for dirt and grease at front crank pulley teeth.

You checked cam timing with timing template when you replaced belt right?

You can check ignition timing with timing light if you chalk mark crank pulley and timing cover to match notch on pulley teeth.and mark on front cover.

It should move when you rev up the engine. Computer senses speed via RPM/Timing sensor fixed air gap.
 
#13 ·
#14 ·
OK,
Here we go.
Re-checked plenum ground connection, replaced EVERY wire connection in the engine compartment, replaced temp switch, airflow meter checks out, rev sensor checks out, re-checked cam timing, Re-checked compression, all great, still no luck. Starting to think I have a clogged cat converter. Kind of beat right now. Going to pull the cat & gut it in the morning.
Thank you, gentlemen, for all of the kind advice and guidance. If there are any more thoughts, don't be bashful.
I will not let this beat me!!!
Again, my deepest gratitude!
Ciao, Al.
 
#15 · (Edited)
You haven't said exactly what the engine acts like when it loses power. Does it start to stutter or miss as the engine temp rises, or does it just feel weak even with throttle open, increasingly weaker as the temp rises? You have to be very specific in order to sense subtle clues which might exist. Much of what you have done is good but I suspect is irrelevant and not related.

Interesting idea about the cat, but usually, I suspect that problem would show up right away, even when cold. Should show up if the engine is revved, hot or cold. That what happened with a Ferrari Carlo was working on. Aftermarket cats failed inside and were clogging the outlet pipes when the throttle was opened, even when cold. Idled fine.

Just a thought, but have you checked fuel pressure when everything is hot and up to temperature after a drive? Maybe the fuel pump fades when hot?

Or, something different to think about. Are the brakes slowly applying themselves, as can, and has happened to quite a few if the seals in the master are failing during a hot day drive.. Had this happen with my 91S. The car would start bogging down, as if the engine was losing power. Replacing the master solved that problem.
 
#16 ·
Is your intake plenum mounted on rubber mounts on top of valve cover and if so do you have grounding strap between a throttle body bolt and rear of head or a valve cover bolt? Also check grounding strap between transmission and chassis near air cleaner box.
 
#17 ·
Brake drag as a cause can be checked by disengaging the engine (neutral or clutch depressed ). Then check the engine will rev to red line with no load. If it will then the brakes are a suspect. If not then it's an engine thing.

Is your evaporative emission control system allowing air into the fuel tank? There is a purge valve on the canister which can fail. If the air can't get back into the fuel tank as fuel is consumed at the correct rate then the pump can't pull fuel at a high enough rate to maintain fuel pressure.

When our car did this there seemed to be a correlation between the symptom and a vacuum developing in the fuel tank. I stopped the car and got out to release the fuel tank cap. Air rushed in and the engine ran normally for a while. Mind you, the problem eventually just went away with no repair effected so I never really did find out. Worth a try.

I think Alfa allows the tank to pressurize as heated fuel is returned to the tank from the main fuel rail. Alfa uses the fuel pressure regulator tank return system. I think every maker now uses the continuous loop system, and DI has to do this. Continuous loop allows fuel pressure regulation to return to the pump intake side directly. Alfa (motronic) still uses tank return for excess pressure.
 
#19 ·
Is your fuel in tank fresh?
 
#23 ·
When it does this and you stop the car, leave it running and then try revving with the gas cap off. As Michael suggested, I wonder if the gas tank is developing a partial vacuum when running. Removing the cap will restore atmospheric pressure.

Please be careful.
 
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