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Discussion Starter #1
1994 NG900, 2.3NA, 5-speed. All of a sudden I am getting a sporadic complete loss of power and revs while trying to accelerate away from a stop. This is happening after driving 20 to 30 minutes in 95 degree weather. There is no sputtering, missing or cutting out, it's just a complete loss of any power from the engine and the revs drop down to idle, even though I've got my foot on the gas. If I let off the gas it will eventually come back.. The engine never dies and I can cruise endlessly once I get going. It only happens when I try to accelerate and then sporadically. At first, it was only when I did 1-2 shift, but today it started in 1st gear trying to start off. I changed the TPS thinking it might have a dead spot, but that made no difference. Plugs, wires, cap, rotor, etc. are all relatively recent. It seems to be heat related, as I take the exact same route coming home from work at midnight when the temp is 70, and have no issues. Anyone had anything similar or have suggestions on where to start. I don't want to just start throwing parts on the car that I don't need. Thanks for the help.
 

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Failing to idle is VERY strange. The throttle is a mechanical linkage... the throttle's opening should stay constant to the pedal's position. Sometimes things like a bad coil or CPS can lead to an engine suddenly stalling, but to actually return to idle speed regardless of the pedal position defies explanation.
 

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Discussion Starter #3
Yeah, hope I explained that right. It never stalls or even runs rough, but while accelerating it's like you hit a brick wall all of a sudden or the engine just completely stopped and the revs drop down to about 1000 (idle level) and there is nothing, no power, no acceleration, no revs, even though the gas pedal is halfway down or more. That is why I initially thought TPS. I can cruise at 55 or 60 all day with no problem. It's only while accelerating.
 

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By and large, the TPS is used for fuel trim... it's a load sensor. The vast majority of air:fuel is handled by the MAF. X throttle opening at Y RPM = Z air mass, and that's how you get fuel. The TPS/ECT/O2 are sort of trim devices, there to take the bulk setting from MAF and improve emissions and drivability. It's not inconceivable, but it'd be really really odd to see a TPS do anything that grotesque.

When your foot is at X% throttle, you should have commensurate engine speed. You control the air, the air controls the engine speed.

Maybe the MAF is misreporting volume and the engine is not really running at idle speed, but lacking correct fuel and thus running at some really low RPM that happens to look like idle speed?
 

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Discussion Starter #5
It does seem like complete lack of fuel, which was my initial inclination. Almost like what used to happen when cars got vapor lock, which I know is not possible with fuel injection. I have two other NG900s. I may switch the MAF from one of those and see what happens.
 

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Discussion Starter #7
After doing some more searching, I'm leaning more toward fuel delivery issue. I'm gonna change the filter first. I found an almost identical post from a while back, exact same model year, manual transmission, almost identical mileage, same symptoms after driving 45 minutes in hot weather. There was no resolution ever posted though.
 

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After doing some more searching, I'm leaning more toward fuel delivery issue. I'm gonna change the filter first. I found an almost identical post from a while back, exact same model year, manual transmission, almost identical mileage, same symptoms after driving 45 minutes in hot weather. There was no resolution ever posted though.
Aloha jotzgr: Did you resolve your issue with fuel pump replacement? I'm having a similar problem. Thanks.
 

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Discussion Starter #9
Aloha jotzgr: Did you resolve your issue with fuel pump replacement? I'm having a similar problem. Thanks.
No I did not. I've pretty much concluded that the fuel pump is probably tired (190K). However since the temperature is no longer 100 degrees every day and is in the 60s and 70s now, I have not had another issue as of yet. BUT, I have not driven more than 20 or 30 minutes each trip either.
 

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Discussion Starter #10
Had no problems all winter as long as temperature was below 50 degrees. Alas, temperature is climbing again and the problem is back. Definitely related to outside temperature. Starts sporadically after driving about 40 minutes at 50 degrees or higher, then after about an hour, every time you try to accelerate from a stop it's like hitting a brick wall: no sputtering, no hesitation, just goes down to idle even though the pedal is pushed down significantly, although it never stalls or actually stops running. I can cruise all day at 60 mph or go up the steepest hill with no hesitation; it's only when accelerating from a stop that it has issues. I replaced the ignition coil yesterday with a new one, and previously replaced the TPS and fuel filter. None of those made any difference whatsoever. Thanks to Mother Nature washing away a four-lane divided highway in February, I now have an almost 2-hour commute each way to work every day. I hate to keep throwing parts at it, so any suggestions/ideas/similar problems would be greatly appreciated. Thanks!
 

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That sounds like a vacuum leak somewhere near the TB to me. I didn't review the old posts, so ignore if you have already ruled this out.
 

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It could be the MAF. I don't think the non-turbos have an IAT sensor, or I'd look at that. I think with Motronic the MAF does IAT since there's no turbo to change the number after intake. Maye the ECT sensor?
 

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surprised nobody has suggested misaligned crankshaft position sensor. Will cause revs to suddenly drop to idle at high temperatures as the CPS loses the magnetic signal and the fuel pump relay shuts off as a result until revs fall and the signal is regained
 

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Discussion Starter #14
I will double check all of those. How would CPS get misaligned? I replaced it in 2011 but if I recall it is held in by one bolt with no adjustment. Thanks.
 

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I'd also be changing the engine temp sensor and checking the wiring. May not be the issue but with an intermittent problem that's temp related, I'd cover it.

EDIT: On the CPS - it's lso a component with known temp sensitivity, particularly when hot. 2011 was a long time ago (although seems like yesterday). I used to think they only aged by miles, but a mechanic I worked with this summer who sees several cars a day says they age by time too. Probably worth changing it. Bosch only (and if you used some other brand, that would be a pointer too).
 

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A car won't idle with a bad CPS. Without a tach signal there is nothing to run the injectors or ignition. Dead CPS = dead engine. There is no way to misalign a CPS. It's a precise fit into the recess in the block. Only goes in one way, only fits in one position.
 

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A car won't idle with a bad CPS.
A car will idle with a badly located but working CPS. It will just near-stall at high revs. Spent ages diagnosing this on my 900 before finding there were 2 O-rings in the CPS hole instead of just one and as a result it was 1 millimetre further from the crankshaft than it needed to be
 

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So if you pop the clutch in quickly, the engine idles and does not stop entirely?

Is the Check Engine light on, or are there any pending codes?

Are you using the stock air box and intake system?

None of your symptoms ring a bell (I had a 2.3 for eight years). However, I'd suggest disconnecting the MAF sensor connector. It won't drive perfectly, and will turn on the Check Engine light in a while, but it should run decently enough to see if the specific problem goes away.

(Driving without the MAF, sometimes the idle speed drops really low, and pedal response is a bit erratic. But I drove the car for over an hour with the MAF accidentally left disconnected and didn't figure it out until when the Check Engine light came on, over 100 km later.)
 

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A car will idle with a badly located but working CPS. It will just near-stall at high revs. Spent ages diagnosing this on my 900 before finding there were 2 O-rings in the CPS hole instead of just one and as a result it was 1 millimetre further from the crankshaft than it needed to be
Okay, but that's not something that randomly happened, that's something someone did (incorrectly). A car won't suddenly grow a new o-ring which then causes it to act weird. The CKP sensor is a VR sensor and thus sensitive to air gap. The sensor placement is designed in such a way that air gap is controlled and reliable for that reason. That geometry won't change by itself.
 

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Discussion Starter #20
Very optimistic that it was the CPS. I replaced the CPS and drove over 150 miles today in 75+ degree temperature. The last few red lights before I got to work I intentionally accelerated much more aggressively than normal (after an hour and a half driving without shutting the car off). Not one hiccup, hesitation, stumble or anything. It was consistently having problems before after 30-45 minutes. Going to give it a couple more weeks and if no more problems, I will buy the $100 Bosch CPS. I temporarily got a lower priced one (Delphi) just to make sure this was the issue before buying the Bosch. The CPS I put in 9 years ago was a brand called IDI made in Italy (not a Bosch unfortunately). Will update after more driving. I'm confident the CPS was the issue, and thankful for all the input given by the forum. BTW, never had any Check Engine light at any point and the CPS would explain the tach suddenly dropping down to nothing whenever it started losing power. Thanks!
 
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