High idle cracked manifold (Page 1/2)
GabeBjerk MAY 11, 09:02 AM
I was wondering if a cracked manifold could change the idle. Its cracked were #1 goes into the main pipe. Its before the oxygen sensor so I didn't know if it could mess stuff up. It idles at like 2000rpms.
shemdogg MAY 11, 09:17 AM
Not an exhaust manifold leak, intake mani yeah. Sounds like u have a vacuum line off or broken

shem
82-T/A [At Work] MAY 11, 09:31 AM

quote
Originally posted by GabeBjerk:

I was wondering if a cracked manifold could change the idle. Its cracked were #1 goes into the main pipe. Its before the oxygen sensor so I didn't know if it could mess stuff up. It idles at like 2000rpms.




The high-idle problem on V6 Fieros is almost ALWAYS the result of a cracked EGR tube.

It looks like this:




... and you generally wouldn't even notice it because it's usually located underneath a heat shield fabric (like this beat up looking one)




But it's almost always here where that happens, and it's like having the throttle body open at all times. The biggest problem is that this immediately causes more air, but does NOT lead to more fuel... so the engine ends up running lean, and gets extremely hot... which almost always leads (during a prolonged period) to cracked exhaust manifolds.

This is the replacement part you need for this (once you confirm that this is your problem): https://www.fierostore.com/.../Detail.aspx?s=67019
... it's $119, and will outlast the original (it's steel braided, not just a steel pipe).


This tube is located just behind the throttle body. It attaches underneath the red intake plenum, and attaches to the EGR which is off to the side beside the starter and ignition coil. Here's a good picture of a Fiero V6 engine that shows the tube well... it's that silver tube off to the left that looks like it's coming off the distributor (which it isn't).
GabeBjerk MAY 11, 09:37 AM
I have the 4cyl
82-T/A [At Work] MAY 11, 09:50 AM

quote
Originally posted by GabeBjerk:

I have the 4cyl




Hahah... shoot. Ok then in that case, it's either something directly on the intake manifold (some huge leak)... or it's possibly leaking from the brake booster. How do the brakes function?

Second question, do you have a check engine light on, and have you checked the codes?
GabeBjerk MAY 11, 10:11 AM
I have only driven it in the yard so far. The brakes stoped it at 10mph but you have to push really hard. And the check engine light is not on.
GabeBjerk MAY 11, 10:50 AM
Before when I pushed the brakes it would run rough and stall.(I took the booster off and put it back on and it stopped doing this) If I let it sit idling for like half an hour it will go down to 1000 it tries to go lower but when it hits 800 it runs rough or stalls. Also if it's at like 2250 I can step on the gas and it will drop to 2000.
82-T/A [At Work] MAY 11, 11:28 AM

quote
Originally posted by GabeBjerk:

Before when I pushed the brakes it would run rough and stall.(I took the booster off and put it back on and it stopped doing this) If I let it sit idling for like half an hour it will go down to 1000 it tries to go lower but when it hits 800 it runs rough or stalls. Also if it's at like 2250 I can step on the gas and it will drop to 2000.




Hmm... I'm trying to think... it's definitely either a vacuum leak, or a problem with something on the TBI unit (like the idle air control valve. or TPS).


Can you tell me what year it is? Is it an 84-86 or an 87-88?


All of the vacuum lines (including brake booster and everything else) attach to the intake manifold. It's all accessories basically, so there's no concern for anything else that affects engine performance other than the EGR. Because you said that after an hour, it tends to smooth out to 1,000 rpms (fully warmed up), it's possible that it could be the EGR valve has failed, and is allowing it to suck in too much air. When the EGR is closed, this would be less of a problem.


Can you try something? Try unplugging every single vacuum line off the intake. I don't have the intake in front of me, but there should only be a few. One large one on the side, and a couple of smaller ones. Plug them with vacuum line gaps if you have them, or some other way to ensure they're actually capped (like a piece of vaccum hose with the end cut off and a screw stuck into it. Whatever you can do to ensure that the vacuum lines / intake is sealed. If you're still getting a very high idle (after say... 5 minutes), then there's probably something else going on.

You'll want to check the seal for the EGR valve, as well as the gasket for the TBI unit. Either could be bad and allowing air into the engine when it should not be.


Let me know how that works. Can you take some pictures too, and possibly a video of the engine starting when cold (with a walk-around)?


Chances are, there's probably a few things going on with this car, especially if it's been sitting for a while.
David Hambleton MAY 11, 12:52 PM
I ran my '84 SE 2.5L 4 speed for years with both #1 and #4 exhaust manifold pipes broken off from the center #2 & #3 section.
It didn't seem to affect idle speed. I wondered if it might affect the oxygen sensor readings for fuel metering.

I left it that way anticipating that removal attempts would result in broken bolts that would be awkward to get out.
It would be a little noisy sometimes when cold, but quieter when warmer as the broken joints would close up.
Last year I had to replace a failed head gasket, so I replaced the 3 piece manifold with a good one while the head was off. https://www.fiero.nl/forum/Forum2/HTML/146435.html
I was surprised that all the bolts came out cleanly, even the ones with the heads practically corroded off. https://www.fiero.nl/forum/Forum2/HTML/146459.html

[This message has been edited by David Hambleton (edited 05-11-2023).]

GabeBjerk MAY 11, 06:43 PM
I tried capping the lines. It went to about 1100rpms right as I started it.(it started way faster too) It's a 87. Also when I move the bolt in the the line it changes the idle little bit.

[This message has been edited by GabeBjerk (edited 05-12-2023).]