'91 N9TEA Bad Cold Running and Idle

[billbranch]

Hello,

Now the car starts instantly, first crank, just idles low, won't take the gas.

Until hot, the idle is still poor, except that the car is driveable.

When hot the idle is too high. The car has adequate but not neck-snapping power.

Of course the normal stuff is OK. New air filter and clean oil. New plugs. Potentiometer is within 2%. New throttle microswitch. New hose at air meter to turbo. The auxiliary air valve is operating.

I believe my next step is to remove the sensors on the thermostat housing and check their operation. Can anyone give me any further direction. Thank you.

Bill

[bill]

Okay, I will bite with a couple of crude stabs.

Um, maybe fuel pressure regulator? How old is it? I just pulled one off that has the original factory clamp, believe it not. When bad, they can do weird things.

The other possibility might be the check valve on the fuel pump. I have seen two bad ones so far.

-Bill

Hello,

Now the car starts instantly, first crank, just idles low, won't take the gas.

Until hot, the idle is still poor, except that the car is driveable.

When hot the idle is too high. The car has adequate but not neck-snapping power.

Of course the normal stuff is OK. New air filter and clean oil. New plugs. Potentiometer is within 2%. New throttle microswitch. New hose at air meter to turbo. The auxiliary air valve is operating.

I believe my next step is to remove the sensors on the thermostat housing and check their operation. Can anyone give me any further direction. Thank you.

Bill

[Bean]

I'd say timing. That can cause the issues you mentioned if not set properly. Since it doesn't run well in cold I'd suspect the temp sensor as well.

Rabin

[billbranch]

I'd say timing. That can cause the issues you mentioned if not set properly. Since it doesn't run well in cold I'd suspect the temp sensor as well.

Rabin

Thanks for the tips. I hesitate to question the check valve as the fuel pump has less than 10,000 miles on it. I shall check the pressure regulator as I'm familiar with the effects of a torn diaphragm.

But just what is the acceptable range of values for the NTC sensor? At 20°C I was reading 4 ohms. At 80°C 0.66 ohms.

Thanks.

Bill

[Palme]

Thanks for the tips. I hesitate to question the check valve as the fuel pump has less than 10,000 miles on it. I shall check the pressure regulator as I'm familiar with the effects of a torn diaphragm.

But just what is the acceptable range of values for the NTC sensor? At 20°C I was reading 4 ohms. At 80°C 0.66 ohms.

Thanks.

Bill

The readings for the NTC sensor should be:

8,200-11,000 Ohms at -10°C

2,280-2,720 Ohms at 20°C

290-370 Ohms at 80°C

All data from: Technical training - System diagnosis

...So your readings are nothing but crap, I dare to say. They are ridiculously, almost impossibly low?

It's a -91 N9TEA you have, right? And the NTC sensor is black? The black NTC sensor is grounded through the threads, and not through one of the connections on the sensor. Therefore the resistance should be measured between one of the connections and ground to get it right.

Regards,

Alex

[billbranch]

The readings for the NTC sensor should be:

8,200-11,000 Ohms at -10°C

2,280-2,720 Ohms at 20°C

290-370 Ohms at 80°C

All data from: Technical training - System diagnosis

...So your readings are nothing but crap, I dare to say. They are ridiculously, almost impossibly low?

It's a -91 N9TEA you have, right? And the NTC sensor is black? The black NTC sensor is grounded through the threads, and not through one of the connections on the sensor. Therefore the resistance should be measured between one of the connections and ground to get it right.

Regards,

Alex

I'm a bit ignorant of where to check the sensor's values, I guess, but I did score a brand new one on eBay for $36.00. I'll drive the thing tonight and see if there's any improvement. And check the timing, although I don't think it would change all by itself. Maybe the valves need adjustment.

[Palme]

...but I did score a brand new one on eBay for $36.00. I'll drive the thing tonight and see if there's any improvement.

Allright, sounds good. Let's hope It will get better.

By the way, now I noticed that year and engine was of course typed in the topic. Careless by me.

Regards,

Alex

[billbranch]

I experienced some bizarre sound effects coming from under the hood last Sunday, nursed the car home, and had it flatbedded to the doctor. Diagnosis: extremely low compression in #3 cylinder, he will do a leak-down test soon. This probably explains the crappy idle and relative loss of power.

Does anyone have a source for these pistons? I'd say "a good used set," but that is what was allegedly in the car when I bought it. I was running at a relatively low boost to try to avoid this kind of thing. Now I see no reason to replace just one: the others can not be far behind.

[Bean]

Best bet to do it right would be to replace with custom pistons. Pretty sure Arias, and JE have patterns for the stock pistons in a modern forged design.

I have good used stock pistons, and even a set of used forged pistons in the stock pile, but when I finally have everything to rebuild the engine properly it's likely getting custom pistons and rods. New stuff is so much lighter and stronger.

Rabin

[Jim in Seattle]

Best bet to do it right would be to replace with custom pistons. Pretty sure Arias, and JE have patterns for the stock pistons in a modern forged design.

I have good used stock pistons, and even a set of used forged pistons in the stock pile, but when I finally have everything to rebuild the engine properly it's likely getting custom pistons and rods. New stuff is so much lighter and stronger.

Rabin

As someone who has installed a set of Arius forged pistons in my project car, I would highly recommend against it due to the excessive noise until they heat up (which can be twenty or thirty minutes to get all the way there) besides being annoying, it introduces a whole new set of slaps into an already relatively noisy motor. Yes, they are peace of mind, but it won't be the same luxurious ride after you do it. Trust me, I know. Stick with Mahle. And if you do build it with the forged ones, make sure you do the custom metal head gasket from Polytecnic and the high strength head studs from ARP. -Jim

[billbranch]

OK. Do the Mahle pistons have a part number or are they a custom item?

[Bean]

As someone who has installed a set of Arius forged pistons in my project car, I would highly recommend against it due to the excessive noise until they heat up (which can be twenty or thirty minutes to get all the way there) besides being annoying, it introduces a whole new set of slaps into an already relatively noisy motor. Yes, they are peace of mind, but it won't be the same luxurious ride after you do it. Trust me, I know. Stick with Mahle. And if you do build it with the forged ones, make sure you do the custom metal head gasket from Polytecnic and the high strength head studs from ARP. -Jim

I was under the impression that running forged was OK, but you had to pay special attention to fitment. IE - bores bored out precisely to fit the piston rather than fitting the piston to the bore if that makes sense. If you get the clearances off just a fraction, or if you use generic clearance specs then it makes the cold piston slap issue more pronounced when running forged pistons.

Maybe piston materials is the difference - but I was sure if built to piston requirements you wouldn't have piston slap issues when cold.

Worth discussing though - as it's good info to sort out.

Rabin

[billbranch]

I was under the impression that running forged was OK, but you had to pay special attention to fitment. IE - bores bored out precisely to fit the piston rather than fitting the piston to the bore if that makes sense. If you get the clearances off just a fraction, or if you use factory clearance specs then it makes the cold piston slap issue more pronounced when running forged pistons.

Maybe piston materials is the difference - but I was sure if built to piston requirements you wouldn't have piston slap issues when cold.

Worth discussing though - as it's good info to sort out.

Rabin

Let me know and I can be the guinea pig. I'm of the mind that I can't go stock unless I am sure no damage was done to the cylinder walls. Right now I am waiting for the head to be pulled. Then we can send a piston off to the shop and have the block rebored and I'll have a new engine. I hope my bailout comes soon.

[geo]

Let me know and I can be the guinea pig. I'm of the mind that I can't go stock unless I am sure no damage was done to the cylinder walls. Right now I am waiting for the head to be pulled. Then we can send a piston off to the shop and have the block rebored and I'll have a new engine. I hope my bailout comes soon.

You're probably going to find that the cylinders are fine and the piston ring landings are cracked. That's what I found on mine when I pulled it apart. There was so little wear on the cylinders that there wasn't even a ridge to cut to remove the pistons - and this was after 175,000 miles! I've been told that there is a lot of nickel in the N9T block so it is extremely strong. Either nickel or diamonds.

-George

[Bean]

Let me know and I can be the guinea pig. I'm of the mind that I can't go stock unless I am sure no damage was done to the cylinder walls. Right now I am waiting for the head to be pulled. Then we can send a piston off to the shop and have the block rebored and I'll have a new engine. I hope my bailout comes soon.

Hi Bill,

I did a fair bit of reading and it does look like that Jim is pretty much bang on. Piston slap can be limited with piston design, and a lot of R&D - but for the most part most guys just live with piston slap on cold start up.

Found a pretty comprehensive piston FAQ here: Piston Info

The race engines I've been around were pretty high dollar builds (Think $30+K) so the piston slap was never an issue even when cold from what I could tell. A lot of that likely had to do with piston design and dialing those clearances into the very last .0005"

Using used pistons is a crap shoot - so I'd call the Madhu at Parts Network as I *think* he had a line on some NOS oversize pistons. Worth a shot especially if you're not going after big power. If you end up using used pistons - I'd look at having them bead blasted and maybe cryo-treated to give them a bit better strength. Pretty sure coating companies won't do used pistons - but the new piston coatings are amazing as well.

Forged is still an option of course as they still are a better choice - but that piston slap at every start up would definitely suck.

Rabin

[billbranch]

Thanks to all, right now I'm waiting for the head to be pulled. I can get stock new pistons which are quite dear but which would cut the turn around on the overhaul by a month. And, no, I'm not after power (speed maybe), this is a nearly twenty year old wagon and it's a long-distance runner, not a sprinter.

[billbranch]

I'll naturally say again "mirabile dictu" because apparently Roger became a bit alarmed and the car really needed a valve adjustment. So now the car will hold an idle and runs smoothly. However, the last time he started it cold outside it wouldn't accelerate, but, with the accelerator depressed, the car would idle like from 1200 to 3000 in a cycle. Has anyone experienced anything like this?

Also, does anyone have a valve cover gasket they'd like to part with?

Thank you.

Bill

[Bean]

Sounds like the TPS... Throttle body? Maybe the AFM? Accordian hose to turbo OK?

I had a Subaru that did something similar and was very bizarre - turned out the engine was way out of time. Set the timing to spec and it was perfect.

When my 89' starts here in the extreme cold it won't take any throttle till it's a bit warm, then it runs fine.

Rabin

[billbranch]

Sounds like the TPS... Throttle body? Maybe the AFM? Accordian hose to turbo OK?

Rabin

Thanks, Rabin. I think it's definitely a mixture issue. Tomorrow he'll be hooking up the smoke machine to the engine.