I have a local buddy w/85 US AT
he's been complaining that it sputters during initial acceleration... he lives in a rural area and since this is a new symptom I thought he may have gotten some bad gas... I swapped out the fuel filter (gas looked kinda dirty/nasty)
it's now doing it again a few days later... he thinks it's spark plugs but I don't
anyone with any ideas before I start tearing into things

the fuel pump is a yr old and works (though it could have sediment in it)
tb etc were all done in the last yr

 

It sounds like you are on the right track if the fuel filter cleared the problem temporarily. You might need to drain the tank and flush the system. If the crud reached the injectors they too might need to be flushed. I think the injectors have fine screens at the top, shaped like a thimble that might need to be cleaned.

 

yeah, I told him I think there's lots of sediment in the lines and everything should be taken off and cleaned, injectors checked etc... car now has about 150k on it, clogged/bad injectors could be the likely suspect

 

My ex-partners 928 did the same thing(his was an 85US).

On heavy acceleration it would sputter or 'buck'.

He never did figure out what was causing it(after much $$$ spent on parts), BUT...when he added an aftermarket Hi-Po chip- it stopped completely.

That was 2 years ago, and so far as i know, the problem has never come back.

 

If the owner thinks it might be spark plugs throw in a set of the basic copper Bosch plugs, that is less than $20. The ignition timing advance is a mechanical vacuum unit inside the Ignition brain and has a vacuum hose from the intake to the brain. The throttle position switch on the throttle body tells the brain when you are trying to accelerate hard so it adds fuel . You stated it sputters on INITIAL acceleration that implies that it runs OK after it gets rolling ? A plugged fuel filter would be restrictive as the engine used more fuel at higher RPM so if it will Rev to 6,000 rpm under load it is not a fuel filter issue. It is ALWAYS best before tuning an engine to make sure there is a GOOD engine to start with so when changing the spark plugs do a compression test. The sputtering maybe a lean running condition which can be several things including a vacuum/air leak , a failing Mass Air flow sensor, the throttle position switch etc..... Try swapping the mass air flow off your S-4.

 

THANKS JB!

he says it runs ok under load, but really sputters initially to the point he doesn't want to drive the car anywhere

I did pick up a set of plugs ($25) and will go ahead and install them and do a compression test...
then check vac lines and swap MAF to see if that's the issue...
man I need a new shop to do personal work on....

thanks again, if anyone else can think of anything else to check let me know, the LH Jet are not my forte

 

The symptoms sound just like those my ex-partner was having.

A failing e-prom turned out to be his problem, as indicated above.

 

The aftermarket chips add fuel and add ignition timing advance under acceleration which perhaps is was what made it run better.

 

Perhaps, whatever the case, it corrected the problem entirely- and gave him a few more ponies to boot.

My kind of repair.

 

thanks but I want to find the problem not bypass it

this guy has gotten ripped off by a couple shops and insists I do all the work on this car...

I think his uncle has a bay available I can put it in for a few days to figure this out

 

ok... compression appears ok according to the guy who checked it (I wasn't there)
however..
there was unburned fuel in 2 plug holes... (center plugs on drv side)
I'm thinking the wires are bad as well, or maybe dist or ign brain

anybody with a diff idea?

 

Yo Rick, how's 'tings?
That sounds like what my car was doing. It felt like it was breaking up briefly when I stomped on the pedal, then slowly cleared as the rpm's went up & took off! I replaced both coils on a gut feeling. The problem stopped completely. But now when I first start it cold it still stumbles for 5-10 seconds as it wakes up & idles but clears & I drive it away. I'm installing a new MAF on Monday. Hopefully that will solve the starting stumble. (I have 175k on an original MAF)
Good luck,
Hammer

 

hey Hammer!
he says there are no problems in the morning when it's cold, it gets bad as the day goes on
could be the fuel building up as the day goes on and he drives it
once warmed it sputters on takeoff then smooths out somewhat as the rpms go up
the wires are old and I'm betting the coils are original
so I guess on Mon I'll order a new set of plugs and coils to boot since they aren't that expensive

 

Check out my various "rough running" threads for more details.

In the end, I found that the connector at my MAF was bad and was corroding constantly; some Deoxit and dielectric grease, along with Bosch platinum plugs in the "9" heat range solved all my problems. Along the way I worked on everything: plugs, wires, T2 sensor, injectors [rebuilt], MAF [rebuilt by John Speake], LH module [found to be incorrect- modified by Speake], new intake seals...etc.

-Bailey gave you a pretty good hint: Tell him to do a good, complete tuneup on the car. Only then should you start start digging into the nits and grits. These cars are quite sensitive to tune, like most '80's high-performance cars, and my experience is that you can recover a whole lot of lost performance just by bringing all the obvious variables back to standard status. Only then will a "weak link" in the chain become obvious.

Happy hunting-

N!

 

thanks Normy
from looking at the plugs and comparing them, he's obviously not getting good spark
he definately needs new wires and and coils wouldn't hurt
I'll take the MAF off the S4 and take it with me to test that if wires/coils don't do it

the car has undergone a full tb/wp/tuneup/belts the whole nine yards in the last year from a reputable shop that I know/recommend... the wires weren't changed I checked the receipts, kinda odd considering the shop
oh well... gonna be busy busy busy the next week or so