Car too weak to redline in 1st, but idles fine. Wtf?

[Earendil]

A friend just swung by with his 91 ix. He says he's had massive power loss, and doesn't think the car can even redline. So I take it for a spin, and here is what I know:

Car idles fine
Car starts up fine
Car functions the same, hot or cold.
When at idle, exhaust doesn't smell like gas (ie normal).
Car feels "okay" at lower RPMs. Not amazing, but passable for a heavy ass stock ix.
Car can NOT reach red line on a flat road in first gear. As the RPMs rise, power smoothly drops completely off a cliff.
There is no hesitation, no dead spots. The car runs smooth.
The engine still sounds "off", especially as the RPMs rise, but it doesn't feel like a car with 3 spark plugs pulled.

Of note, routine+ maintenance was done last spring. plugs, all filters, etc.

So what do you guys thing? Fuel starvation? Could the spark be failing under heavy (high RPM) load? I've never heard of or experienced an issue like this before.

When he brought it by it was dark. Saturday I'll go by his place and see what I can do. But I want to be armed with as much info and trouble shooting techniques as I can get, because neither of us has a garage, and we have highs in the 30s, and lows on the 20s these days. I can bring my fuel pressure gauge, but that's one of those tests I'd rather not do if you guys can help me avoid it!

Thanks for any help!

Cheers!
(sorry for any typos. Written on my phone)

[jlevie]

Check the WOT switch in the TPS.

[Earendil]

Check the WOT switch in the TPS.

What would I be looking for? My understanding of the m20 TPS is that it has three positions, closed, WOT, and "in between". If it were getting a constant closed signal could that really keep it from reaching redline in 1st?

At least that's a test I can do from inside the car! Thanks!

[russhixmo]

My e28 did the same thing but it wouldn't rev past 4k. Mite be a clogged cat 😱

[ROLLingKING]

what exactly was changed last spring. etc does not equal the same tune up as another persons.lol idk much about working on an ix but i may be able to suggest something if i know what hasnt been swapped

hope it gets fixed soon. no powa is no fun!

[Killacortes]

I had a friend lose a lot of power after a new exhaust installed. Was a unplugged 02 sensor. Just sayin

[AndrewBird]

Will it reach redline if you accelerate slower, say, at half throttle instead of full?

[jlevie]

What would I be looking for? My understanding of the m20 TPS is that it has three positions, closed, WOT, and "in between". If it were getting a constant closed signal could that really keep it from reaching redline in 1st?

At least that's a test I can do from inside the car! Thanks!

You are looking to see that the WOT switch signal goes true when the throttle is 60% or more open. Use a meter and check at the DME connector.

[pharow]

I had a problem like that. It was my fuel pump. Check fuel press. to see if it drops off. If it does, most likely it's the fuel pump.

[603Racing]

I've seen a severely worn cam with weak springs do this exact same thing. Car wouldn't go past 5k RPM at WOT driving down the road. Replaced the cam/rockers/springs, and it ran like a champ.

Shot in the dark here... MAF flapper sticking or not opening all the way?

I also agree that it could be fuel pressure or TPS.

[Earendil]

I want to start by saying that I won't be dismissing anyone's advice...okay maybe a few people ;-)
But I'm an engineer, I'm forced to question all information I hear. Plus I like learning :D

what exactly was changed last spring. etc does not equal the same tune up as another persons.

The ix was bought last winter. I picked it up for $1500 and flipped it to a friend who isn't as mechanically inclined for a small profit.
I did a lot, but engine wise:

• New plugs
• New fuel filter
• New Air Filter
• New oil filter + oil
• Coolant flush
• New Coolant temp sensor (I had a spare, and his was questionable)
• New O2 Sensor
• New Timing Belt and water pump
  
• Took everything upstream of the intake manifold off and cleaned it, inspected for cracks.
• Adjusted the TPS, it wasn't detecting WOT.
• Adjusted the valves.
• Checked every sensor on the car to make sure it was within spec (how I found the CTS and TPS).
  
• Everything downstream of the headers was replaced 3 years ago.
• The plug wires were also supposed to have been replaced 3 years ago, but they look cheap and like they've been there for more like 10 years.

So it was rather disheartening to find the car unable to pull past 5k. I've suspected that the car might have a vacuum leak someplace. But would a vacuum leak cause the car to lose power with an increase in RPM? Every leak I've ever had hasn't, but I've also never lost that much power.

I had a friend lose a lot of power after a new exhaust installed. Was a unplugged 02 sensor. Just sayin

I'll see what that does for me, but I used to run without my O2 Sensor all the time when I was trouble shooting some other stuff. Mild power loss, nothing major.

Will it reach redline if you accelerate slower, say, at half throttle instead of full?

Nope. Once you are above 3K the car has almost nothing left to give, at any throttle amount.

You are looking to see that the WOT switch signal goes true when the throttle is 60% or more open. Use a meter and check at the DME connector.

Is it really supposed to go WOT at 60% or more? I'm not sure you can detect closed if it's adjusted in that way. You have some impressive titles in your sig though, so I'll double check :)
Yep, the DME connector is the way to go! especially when it's cold outside! :D

I had a problem like that. It was my fuel pump. Check fuel press. to see if it drops off. If it does, most likely it's the fuel pump.

I'll certainly do this. I once had a grounding issue on my fuel pump. The pump was fine, but the ground was severed. The pump was still grounded enough into the frame to operate fine below 3K, but it felt more like 100hp up high. It would easily climb to 6K in 3rd gear though. I suppose I'll be digging out my fuel pressure tester after all. Thanks a lot!

I've seen a severely worn cam with weak springs do this exact same thing. Car wouldn't go past 5k RPM at WOT driving down the road. Replaced the cam/rockers/springs, and it ran like a champ.

Shot in the dark here... MAF flapper sticking or not opening all the way?

I also agree that it could be fuel pressure or TPS.

Don't tell me cams/rockers/arms.... please....
What other symptoms might that show? Seems like the car would run super rough if that were the case. But maybe not?

I'll check the AFM flap and make sure it's good. But I did clean it 9 months ago or so. And fuel pressure, really? That's the one thing I don't want to do outside in the cold! :curse:
Oh well, at least it's supposed to be sunny!

Thanks for all the tips guys. More advice is of course welcome. I'll be taking a look at it Saturday morning and I'll have something for you guys by Saturday night!

Cheers!

[Earendil]

Update. Car is down for the count. Friend was driving it home from work, and it suddenly started making a ton of exhaust noise and he lost all power. I go pick him up and take a quick look. I find there is an exhaust nut missing, and there is a 1/4 inch gap in the exhaust manifold to downpipe. Park of the gasket has broken and been blown out of the new gap.

Could a giant hole like that be the cause of all problems? Or is this just stupid and dumb luck?

[jlevie]

The exhaust leak won't cause the loss of power experienced, though it will make a lot of noise.

[Cletonius]

The exhaust failure may point to a plugged cat. I had a similar issue on another vehicle in the past.

[Earendil]

The exhaust failure may point to a plugged cat. I had a similar issue on another vehicle in the past.

How does one test a plugged cat? Also, the cat is only 3 years old...maybe 2.

I've eliminated most of the things on our list. Fuel pressure is great, TPS is spot on. Nothing obvious wrong or out of place. Popped the valve cover off and didn't find buys of metal. All the springs (inner and outer) are good. The spark plugs looked good. If anything, they might look sort of Lean, but not terrible.

Exhaust leak is fixed, so we're back to where we were a couple days ago. Next up is a compression test (forgot the gauge yesterday), and to see what the system voltage is while idling and under strain. I'll also look at the cap and rotor, even though they aren't even 9 months old. Maybe it'll at least give me a clue...

Another though... I believe I've read that being a tooth off in the timing belt "won't cause damage, but it'll run like shit". If the belt jumped a tooth, would it keep power down low and go dead up high?

I'll report back in a bit.