Mad993

*in the end a fuel pump and fuel regulator fixed the issue*

Hello ,
Question for the expert mechanic on here.

My 993 was ripping it pulled like a freight train to redline. Then all of a sudden in first or second gear at high rpm it backfired though the intake and died. Started right back up but now runs like in limp mode. Smooth idle and good midrange power but nothing on top and it has me puzzled.

Car does not want to rev past 5500rpm . Car is stock obd1 non verioram.

I figured in was was getting close to the 60k mark so I went ahead and did the 60k Service it needed it anyway.
This is what I have replaced so far.

-spark plugs Bosch
-fuel filter Mahle
-air filter Mahle
-distributor caps Bosch
-distributor rotors Bosch
-02 sensor Bosch
-cleaned isv
-replaced maf
- new ignition module
​​​​​​-air temp sensor on intake
- throttle position sensor
- new plug wires beru
even tried some fuel system cleaner.


Here is what I have checked.
-I pulled the coil wire to the distributor caps car runs on either distributor alone but, rough on secondary distributor.( is this a proper test of coil and ignition module * could I have a problem but only at say 5k rpm?)
- checked secondary rotor could not spin so distributor belt is not broke?
-checked the three vacuum actuators no leaks.
trace my vacuum lines all look in order. Car holds vacuum hours after shutoff.
-tested my intake flap it’s working on key on and startup.(but no idea at wot)
-no check engine light.
-checked plug wires look ok not terrible not perfect

*so so after the tune up the car idles and runs sooo smoothly clean up thu midrange actually I gained a lot more midrange power but lost the top end somehow just feels flat lost it’s punch if you know what I mean.

I noticed the fuel filter was quite dirty
maf was filthy * if my maf was bad would it throw a check engine light? (Went ahead and replaced it .)

could my fuel filter be clogged again? (Replaced it again)
weak fuel pump at 55k???

check my cat converter with inspection mini camera and catalyst looks* ok no build up .

thanks for reading any suggestions would be appreciated.
thanks!

pp000830

Consider running the OBD Codes and see what comes back.

Mad993

Do you mean key on holding gas pedal down and counting the flashing light ?

It came back no codes

also I should clarify it starttruning bad before I did the tune up

TheOtherEric

First thing I’d do, before spending hours and hours and several hundreds of dollars, would be to spend $200 on a dyno pull to see if there’s really an issue. Could be all in your head.

pp000830

I was thinking to use the diagnostic port with the correct round to square adapter if needed and an OBD Code reader.
Looking at running data may reveal something as well. The fuel filter is rather high capacity so I doubt it is the issue. The butterfly in the throttle body or any related sensor on it could be out of adjustment where it is not fully mechanically opening. this seems reasonable if the car runs well in all other aspects. I would think a clog or restriction in the fuel system would result in a lean condition at near wide open throttle and would cause some sort of diagnostic code related to lean misfires.

I am not fully versed on the non-Varioram cars. There is an excellent paperback book published by Bosch on theory and diagnosing all versions of their fuel injection. It's inexpensive new or used on Amazon or eBay.
Hope I helped here,
Andy

Mad993

Thanks Andy
Is there any adorable code reader/ diagnostic tool you would recommend?

so you’re saying a code reader may show miss fires but not throw a check engine light.

I have read about the butterfly valve % and gas pedal cable not fully opening I’ll look into it good idea!

When I changed my filter there was some very small rubber chunks when I dumped it out into a bucket. I attached a picture of it what do you think?


*Also my plug wires are original yes some of the lower ones have small cracks and a dry rotted look to them probably from muffler heat , could this be the issue and only arise at high rpm?

pp000830

Is there any adorable code reader/ diagnostic tool you would recommend?So you’re saying a code reader may show miss fires but not throw a check engine light.I have read about the butterfly valve % and gas pedal cable not fully opening I’ll look into it good idea!When I changed my filter there was some very small rubber chunks when I dumped it out into a bucket. I attached a picture of it what do you think?Also my plug wires are original yes some of the lower ones have small cracks and a dry rotted look to them probably from muffler heat , could this be the issue and only arise at high rpm?Good luck with this,
Andy

Ivan J

If everything else checks out ok, you might have a clogged catalytic converter. Perhaps you could borrow a bypass pipe from someone to see if it makes a big difference.

Steve Weiner-Rennsport Systems

Check long term fuel trims to see if its lean or not.

Replace those plug wires!!! (LONG overdue)

Don't forget about the in-tank fuel filter if you've seen any evidence of fuel contamination. I always cut open the old (main) fuel filter for inspection.

Mad993

I don’t think because this happened all of a sudden one day after some hard hi way on ramp pulls to red line

Mad993

ok will do I thought because it only had 55k they would be ok I didn’t factor in the age of rubber !

pp000830

Learn something new every day, didn't know there was a strainer in the tank:

TheOtherEric

Does anyone have experience checking this? Sounds fascinating. I use a Scantool / raycm interface so I assume I can do it, but don't know what to look for.

pp000830

I believe running data from your oxygen sensors will reveal if the mixture is correct. Durametric and certain other OBDC readers can display the oxygen sensor running data.
Andy

Bill Verburg

I'd also look for vacuum leaks, mine lost ~50ho at the top end because of a vacuum leak in the cabin air handling system up under the dash.