I have a 1996 850 GLT automatic non turbo its my dads car that he doesnt care for but i want to keep it running. I was hoping someone would be able to help with some information on an issue I'm having.

It has an issue where it will rev up above 1000 rpm and then drop down to 500rpm or bellow, then it will continue to go up close to 1000 rpm then down. Sometimes it dies when going down. this will happen randomly, sometimes off cold start, sometimes after driving for a while etc. sometimes i can go a whole drive without the issue. its idle rpm is a bit high, close to 900 rpm but it does have the Lambda light on ( i suspect something with the exhaust but i will get to that soon). I have cleaned the OEM idle air control valve since it was very filthy and that seemed to help the issue, after driving the IAC would become extremely hot and i thought maybe that was from a short in the internal electrical. I bought a aftermarket IAC online and it seemed to be working better the issue went away for a bit. But the issue seems to still happen randomly although not as often. the aftermarket IAC still gets extremely hot after even a 20-30 minute drive.

More about the exhaust and lambda light: the exhaust is very rusted, it was redone quite a while ago(10 ish years). but i don't think the cat was replaced and the mechanic that did the exhaust was a bit of a shmuck. I have a few thoughts about the exhaust being connected to the lambda light. possibly the O2 sensor needs to be checked/replaced. Maybe the cat is clogged and that's causing some issues. I know the exhaust is rusted and has some leaks, that is after the cat, but that could also be contributing the lambda light.

More about the idle and revving: some more things ive noticed is the idle will rev up about 1500 rpm when going from drive(brake pressed) to neutral. lets say i come to a light and i hold the brake down in drive, the idle will be around 700-900 rpm. if i put the car in neutral while stopped the rpm will shoot up around 1500 rpm. the same happens when you go from drive to park idle will be close to 700-900 rpm and once put in park it shoots up to 1500 rpm. takes a minute or two to drop down to 900-1000 rpm where it will stay.

another odd thing about the idle. one time after a start up the idle was sitting around 900 rpm and it then started to do the issue of revving down and up. when it would drop down the transmission light would turn on until the rpm went above like 500 rpm. another time while driving the transmission light was flashing and when coming to a stop the transmission wouldn't down shift to 1st gear. the car had the transmission rebuilt twice about 10 years ago. first it went when driving on the highway just out of no where, it was rebuilt but shortly after the transmission light started flashing and so my dad brought it back and after some work they said coolant was leaking into the transmission. so it was fixed again.

Further more when working on the car myself with an old friend who is a mechanic we were checking around the throttle body/ intake area. we noticed the hose that connects from the crank case to the flame trap into the air intake was really not secured well(possibly from previous shmuck mechanic) we thought that could have been causing the lambda light and vacuum leak. we kind of mcgievered a solution so the flame trap would be more secure. I did a glove test to check the PCV and it did fail which is expected because the whole PCV system was never redone.( might do that at somepoint,)

The issue is random. unfortunately i dont have a scanner that can pull codes but im looking into getting one maybe that will help with the lambda light and such. hopefull someone can help with some info it would be much appreciated.

 

I'm not sure what to make of this IAC getting hot. I don't think I ever touched one to see if it was hot. The car clearly is not controlling rpm properly, but it's not obvious to me why that is. It does not sound like a vacuum leak to me.

 

You can borrow a scanner from most of the big box stores - Autozone etc, plus a $29 unit from Harbor Freight will do fine for reading engine CEL codes. There's a few things that can contribute to a wonky idle that will also set a CEL. Generally its not the O2 sensor even though most code diagnostics will suggest that. Start by inspecting the vacuum lines. Take the cover off the throttle return spool and see if the vacuum tree has any broken hose ends or open ports. PS - if you have a clogged PCV system, the pressure can pop off the caps that seal the unused ports on the tree. You may even want to check our the Robert DIY Youtube vid on how to make a home made smoke tester since some of the lines can be hard to follow. Also make sure the intake air boots are solid and dont have cracks. You can also spray carb cleaner around various seals like the injectors and intake manifold gasket and air boots/vacuum lines to see if anything gets sucked in and bumps the idle. This includes the brake booster and vacuum pump lines. The hardest line to diagnose is the Evap cannister line that runs from the purge valve (front right on the side of the radiator, all the way back to the charcoal cannister. Next check for exhaust leaks ahead of the cat. Not uncommon for the flex pipe and EGR tubes to fail and cause a leak that will set off an fuel trim error (due to an inaccurate O2 lamba response). One thought - if your idle air valve was gunky, consider cleaning the MAF (use MAF cleaner not carb cleaner) and do a rubber glove test to see if the car has a clogged PCV or a failed oil separator which could lead to oil getting into the intake dirtying the MAF and throttle body. After that, I'd probably do a stage 0 tune up - new plugs, check the date code on the wire (if they are older than 5 years, replace), rotor, cap as well.

Note, If the PCV system is clogged, the repair cost for a shop will be in the $600 range. If you have skills and tools you can buy a parts kit for about $200 - you need to lift up the intake manifold to access the hoses and you'd want to rod out the ports into the block or drop the pan to degunk the lower end to do the job right. Given the car is probably worth $500 to $1000, the repair costs can rival the value of the car unless you do the work yourself as a hobby car.

Post up how you make out on the first step of looking for intake air leaks etc

 

that’s super helpful info, I did a glove test a while ago and it failed so I was aware about the pcv. Kinda putting it off but I know I should do it. All super helpful stuff I’ll look into.

I did a vacuum leak check around the same time as the glove test and the flame trap turned out to be loose, the hoses that go from the engine to the flame trap were too short so I kinda frankenstined that to make sure it stays, probably gonna end up doing the whole pcv system and vacuum lines though since the car is 25+ years and that’s never been done…

I’ll keep posting as I work on it thank you so much for the info