Saab 900i hesitates on acceleration

First thing would be change the plugs.

MrCheerful

Agreed, though should have mentioned I've been told by the previous owner the plugs were changed less than 6 months ago at its service... still worth looking as I didn't do it myself, possibly wrong plugs?

Firstly enusre all the ignition HT components are in top notch order- especially rotor arm, dizzy cap, plugs, gapped correctly etc.

Tim..

Will do, thanks.

Seems to be a common problem on many older turbo cars.

Could be lots of things. Ignition, fuel, inlet hoses, turbo.

Ignition, usual suspects, plugs, leads, cap, coil (or coils in case of direct ignition). Gets expensive to replace coils and ignition components at random if not faulty. Check voltage even a little bit down could reduce spark.

Fuel, leaky injectors, fouled injectors that won't flow peak demand, filter, inner liner of fuel hose damaged restricting flow, filter (also in injector inlets) but none of those will get worse with heat except possibly hose liner gets more pliable and blockage gets worse. Possible tank vent problem, I have heard of tanks being sucked flat. The pump maybe on it's way out, heating up and getting worse the longer it is run. Need a high pressure fuel pressure gauge (0-100psi), check pressure at as many places as you can find hose sections to 'T' into. I can't recommend running the fuel line into the cockpit so you can see the gauge while driving so it will need a rolling road to load it up. Really don't even think about it, if it leaks the next electrical contact you make (like the brake light switch) could make a spark.

Inlet leak gives way over rich mix on high boost pressure, lean when off boost. Need a low pressure fuel pump / vacuum gauge (-30in Hg -

20psi). disconnect inlet duct from turbo outlet and throttle plate, plug ends and any other tapings. Fit a schrader valve and pressure gauge on a 'T' from a tapping in one plug or other convenient point and use a tyre pump to pressurise it. Stop at normal boost pressure and check it can hold this pressure. When you have done with the gauge it makes a good boost gauge if you don't have one.

Turbo. Saw this one on a MR2 turbo page. Worn compressor seal is leaking oil into inlet duct, there will be small quantities of clean oil in low points of inlet duct between turbo and plenum. When you floor it the air flow picks up the oil from the intercooler, duct walls etc and dumps it into the engine. Oil promotes knock, so knock sensor retards engine or it gets on the plugs and causes a misfire. When hot more oil gets flung past seal and hot air in duct makes oil on duct floor more fluid and easier to pick up. The MR2 owner said he had run like this for over 25K miles but you don't buy a turbo car to be gentle or have limited and variable acceleration. Check for fresh oil in inlet plumbing by swabbing though with kitchen towel, should be none. (After cleaning problem disappears for one CXXXXIImph run - returns next day + if caught you make front page of the Sun) When cold take inlet hose off turbo compressor, grasp nut and check for play. If you can feel play axially or radially then turbo has limited life. it's re-buildable (new bearings and seals about £200 + core balance + labour) up the point where the compressor makes contact with the scroll, then it's a new turbo and intercooler (can't be 100% sure it's cleaned out) + lots of work stripping and rebuilding the engine as the alloy shavings will have gone though it.

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Peter,

Its a 900i 16V. No turbo on it ;-)

Probably dirty AMM or failed coolant temp sensor leading to overfueling, normaly shows up as a stumble, if left long enough can soot plugs leading to a misfire under load.

Cheers Matt

As **nobody** said, it's not a turbo. However, it may have CAT since Saab introduced these about 1990. Then the CAT could be blocked. Johannes

Blocked Cat would not cause stumbling under light load- it would cause a drastic inability to produce power when asked to though- i.e. with a wide throttle.

Tim..

Thanks for the pointers, all. No turbo on this car, as correctly pointed out. I'll begin with checking the plugs, leads, rotor, etc. to eliminate obvious problems.... it's difference between the smooth(ish) running when cold vs. hesitant when hot that's bothering me.

Don't think there's a Cat on it, BTW.

The reasons for the cold to hot running differences are easy to explain. A rich mixture (cold conditions) is more easily ignited by a poor spark than the weak mixture (hot condition)

MrCheerful

Gotcha... I'm getting ahead of myself blaming coils and the likes before first doing basic checks, like checking condition of plugs.