88 928 S4 Auto - been sitting for several years. About 118K miles. Wouldn't start - diagnosed the fuel pump. Replaced it and the engine starts but is rough throughout the rpm range. Where to start?

Thanks,
Steve

 

A tank of fresh gas, a can of techron, clean or replace plugs, and check for fireflies by spritzing ignition wires with water after dark.

More importantly, what is the condition of the timing belt and has the end play been checked. Both can lead to TBF of different sorts.

 

It can be MANY things...

- Bad coil(s), spark plug wires, spark plugs...
- Bad MAF...
- Bad injectors...
- Etc...

I'm sure somebody here will give you many other things to look at.
Welcome to Rennlist

 

Not sure why others haven't chimed in, but truth be told it can be many things, that is true. Where to begin was the question asked.

Without more specifics, I'd start with the easiest which is why I suggested fresh fuel and techron to give the injectors a cleaning although I'd equate dirty or faulty injectors more toward sluggish performance which is more analog in nature.

The term 'rough' makes me think ignition as in not firing on all cylinders or coughing/sputtering which is more digital in nature. That can also be a lot of different things, again focusing on the easiest first... plugs, wires, coils. Hence water spritz suggestion.

MAF would be further down my list because they have a tendency to go slowly, as in still work, but out of calibration. The out of calibration is related to fueling (analogish) and not ignition (digital) where improper fuel to air (unless extreme) is more like to effect performance.

Getting to the root of the problem is typically and logical sequence of tests; which is hard to do with only one symptom (running rough) and a couple conditions (all RPMs after period of inactivity). I’m assuming this is while sitting still and not driving (to eliminate drivetrain from the equation).

Finally, as I'm sure most will agree a rough running engine is better than an non-running engine. Hence my comment to first start with timing belt (is that a known condition or not) as well as crank end play (since it's an auto). TBF is an acronym used to represent both timing belt failure as well as thrust bearing failure. These two failure modes are not related, but both result in catastrophic engine damage.

So in my mind, the answer to the question of where to start would be... where are you related to TBF avoidance? Second would be to focus on ignition unless another clue indicates otherwise.

As Paul Harvey would say, "and now you know the rest of the story.... good day!"

 

Thanks for the advice. Probably start with new plugs/fuel... will let you know if it makes a difference. FYI belt is fine

 

Why was it parked up?

It may be stuck in 4 cylinder mode, they couldn't figure it out so just left it.

If it was my car I would jumper that relay next to the CE panel first, it's quick and easy. I would take a picture but I'm away from my cars for a month

 

I will start with fresh gas and then go over ALL the electrical, grounds, connection, tune up, timing. If that works, change all the fluids and belts, water pump, thermostat.....