93 Normally aspirated with 171,000 km and original everything as far as parts go. It ran pretty darn good with all that stuff and i decided to give it a tune up. Took it to a mechanic for a new timing belt and water pump. Still ran good. changed the oil and fuel filter, still good. Then the fuel pump went, got a new one and it ran ok as before.
I decided to start with the Cap and rotor that I bought from FCP and is made by Bosch. I think these are the factory parts:
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I changed that over and it ran good until I topped up the tank. It was ¾ full and I just wanted it full to check what fuel mileage I was getting 22mpg with that cap and rotor.
After a short drive I started to feel the engine stumble. It would run fine at a cruising speed 60mph but if you stepped on it then it would like misfire on a few cyls but still accelerate. Idle was also no longer smooth. It is like the car is being turned off for a milli second. I thought it could be water in the gas so I bought a litre of methyl hydrate and put it in. It seemed to improve it as the idle got better but it still craps out at full pedal. I can rev the motor all the way to red line if I do so in a smooth controlled constant manner.
I can accelerate the car the same way to well over 80mph. If I step on it hard it craps out. I left it over night and it seemed a bit better in the morning. It idled better and also ran a little better.

Today I thought it might be the fuel pump pick up so I took it back out checked it and turned it 180deg, nothing(everything was clean as new). Tried the pressure relief valve and it came out strong and checked out at 40psi (not the most accurate gauge). It has a new fuel filter. I tried the pump relay and jumped it, no change. Matter of fact it made it worse again, back to what it was yesterday.
So I will now put that old crappy cap and rotor back in and see what happens. If that fixes it I'll eat my shorts.
Any other thoughts and comments welcome.

 

Do you have a CEL?

 

No
But I can and will check for codes later today.

 

You may need to change the plugs/leads as well. Putting a new cap and rotor on may be stressing out old leads.

If I ever have to change the rotor and cap, I change the leads at the same time so It's all new from the coil to the plugs.

 

Yesterday I took the intake system apart to the throttle body and cleaned it over a 2 hour period. Used up two cans and also cleaned the MAF a second time.
After that I got a 1-2-1 CEL code. I cleared it and had to go to work. Started it again this morning and it now runs smooth on idle and will rev smooth as long as you do not apply a lot of throttle. Then it starts to stumble.
No CEL came on so I assume there are no new codes.

No change from changing the plug wires either. Consider that the car ran fairly well before I started all this tune up stuff. None of these problems existed or at least not to a degree where you have to fix it like I do now.

By the way if I were to buy the plug wires again I would not buy a Bosch set, the quality is just not there like the factory set.

I found this site today: http://www.autotap.com/problem4_engine_hesitates.asp
The next thing I'll try is to clean the fuel injectors.
Also borrowed a friends Fuel pressure gauge and gave it a try, 50 psi.

 

Plugs = Volvo
Wires = Bougicord
Cap = Bosch
Rotor = Bosch

 

I solved my problem after looking through a lot of possibilities. Maybe not as many as others have had but still. On top of what I did previously I checked the fuel preasure at the rail and did a vacuum gauge test.
I really wanted to know just what the exact reason was for the car running so bad and which part it was. Spark plugs. I pulled the original ( They are Volvo and look like they have gone 171,000km by the worn electrodes.) plugs to find rounded and thin electrodes. They were still a nice tan colour which would explain the 22mpg before I started this tune up.

I was wondering what it was that made it change with in a very short 20 km. Plugs do not change gap by them selves but can break or die. These have a gap as thick as a penny 1.55 mm or double the norm.

I think it was, in part the Seafoam I ran through the tank and then sprayed into the throttle body 2x. It cleaned off the carbon to some degree and that could have reduced the gap. The new plugs used are Bosch Superplus with a .9mm gap. The torque value is 21 Ft lbs or 28 nm in case anyone is looking for that.

I almost want to re-gap the old plugs a try them out. Need to test the MPG again now.