93 Z34 - High Idle & Hesitation Issues

[93luminaz34]

Hi. Im new to this board, Ive been reading it for quite a while now& find it to be very helpful. About a month ago I bought a 93 Z34 Auto with 168,000 km on it.

When the engine is cold, it fires right up, idles erratically, with the RPM's slowly increasing till about 2000-2500rpm. I've even had it bounce off the rev limiter a couple of times. When the engine is warm it usually wont start unless I use the gas pedal. During acceleration the motor hesitates between idle right up to around 3000rpm but when cruising along the car runs really well.

The parts Ive replaced to try to correct this problem are:

Intake Manifold Gasket, Plenum gasket, EGR valve & gaskets, Spark Plugs & Wires, Thermostat, IAC Valve, TPS, PCV Valve, Vacuum line from plenum to Master cylinder, Fuel Injector O-rings, fuel filter and air filter.

I've also cleaned out the EGR port in the plenum.

I've had another ECM tried in it at a shop and it made no difference.

The previous owner said that he & a friend changed the timing belt themselves. Is it possible that they didn't achieve the correct timing when they put the new belt on, causing these symptoms?

I also haven't changed the O2 sensor, so it could be a possibility. Maybe a clogged cat? I don't know what to do next.

Thanks for taking the time to read my long post. I'd appreciate any input you have.

[grandprixgtp_91]

my brothers automatic grand prix would do the exact same identical thing when the timing was wrong on it. but we left the timing wrong for a while because it was a quick fix so he could go to work and we just never had the time to fix it. but id re-do the timing and replace the belt dont re-use that one, same goes with the idler pulleys.

Jake

[ryan.hess]

what happens when you pull the vacuum hose to the brakes, and plug it? Does the idle drop to normal? Could also be the gasket around the MAP sensor

[93luminaz34]

what happens when you pull the vacuum hose to the brakes, and plug it? Does the idle drop to normal? Could also be the gasket around the MAP sensor

Yeah I'm going to try plugging that hose today. If it doesnt work out Im going to make an appointment at my dealer to get it diagnosed tomorrow. I have a feeling that its probably bad timing to blame.

[93luminaz34]

I unplugged the brake booster vacuum line and it made no difference so I guess its safe to say that a leaking master cylinder is out of the question.

[ryan.hess]

Lets see... Could also be a massive intake gasket leak. You can check that by spraying some carb cleaner around the gasket to see if the idle changes... You could also have a leak around your EGR gasket (check the same way) - this is more common. To have it hit the revlimiter would take quite a bit of a leak... Does it lope a lot? Seem like it's firing on only a couple cylinders (shake the car?)?

[93luminaz34]

I changed the intake manifold gasket, plenum gasket & both EGR gaskets & nothing made a difference. I tried spraying brake cleaner around all the gaskets before replacing them & found no increase in idle. It idles erratically but is firing on all cylinders. When driving, from around 3 grand & up you cant tell there's anything wrong. It hesitates during acceleration from idle up to 3000 rpm. Like, when you hit the gas from a dead stop the motor almost sounds like it bogs down and then like a couple seconds later the power comes on. The car is running really rich, especially at the lower rpms. Could that have anything to do with timing? Thanks for all your replies.

[ryan.hess]

I changed the intake manifold gasket, plenum gasket & both EGR gaskets & nothing made a difference. I tried spraying brake cleaner around all the gaskets before replacing them & found no increase in idle. It idles erratically but is firing on all cylinders. When driving, from around 3 grand & up you cant tell there's anything wrong. It hesitates during acceleration from idle up to 3000 rpm. Like, when you hit the gas from a dead stop the motor almost sounds like it bogs down and then like a couple seconds later the power comes on. The car is running really rich, especially at the lower rpms. Could that have anything to do with timing? Thanks for all your replies.

I still don't quite understand what you mean by "idles erratically", but.. It could very well be the timing. If one bank were off by 180*, it wouldn't let you get past 3000rpm, so that's probably not it. If the complete system was off by 24* (one notch), it could basically, when floored, kick in knock reduction which would feel like instant limp-mode. It would also lope pretty good. Unfortunately, the only way you could check would be to pull both covers (and the intake and serp belt, etc), which is why you'd want to check everything else first...

[93luminaz34]

Basically I mean that it wont hold a constant RPM. When I start the car, sometimes it will want to rev right to the limiter,and other times it will rev at 900 rpm. Then all of a sudden it will just begin to idle up all on its own, to 1500, to 2500 or whatever it feels like doing. One thing i noticed is that if the cooling fan comes on when its idling normally, it will cause the idle to spike up & stay up. Putting the car into gear and then back into park again also causes it. Sorry if this doesn't make sense.

[ryan.hess]

Does how warm/cold it is affect how far it revs up? Like when it's warm, does it shoot up to 2k, vs cold 3k? If it's not loping, I think the cams are probably fine.

[93luminaz34]

I got the car fixed now. it was a combination of a screwed up Power Steering pressure switch & oxygen sensor. It even passed emissions testing perfectly The only thing that it still does is seem to lack power when taking off & accelerating up to 3 grand. After 3000 it feels good. It wont even spin the tires from a stop.

[Cotton]

I got the car fixed now. it was a combination of a screwed up Power Steering pressure switch & oxygen sensor. It even passed emissions testing perfectly The only thing that it still does is seem to lack power when taking off & accelerating up to 3 grand. After 3000 it feels good. It wont even spin the tires from a stop.

timming is off.

[Crimson Fury]

is your top engine moun's bushings deteriorating?

[93luminaz34]

The rubber bushings dont look the best on the top engine mount. When you grab the dogbone and push & pull on it, you can feel a bit of play in it. Would that also cause a rough shift into reverse from park or neutral?