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Discussion Starter #1
I will try to keep this as brief as possible. I need some advice on an issue I'm having with my 06 sti.

About two months back I spun a rod bearing. Long story there, but it was a pickup tube failure followed by a bad rebuild by a subie dealership that ultimately led to a starvation issue that caused the most recent rod bearing failure.

So, I had my engine rebuilt again (this time out of warranty $$). Break in period went fine (engine purs) until literally 1 week before my final Oil change to end the BEP. I started noticing a serious loss of power in low rpms. That transitioned into a nearly complete loss of power under 3k rpm and severe turbo hesitation, but once the turbo kicks in, it really kicks in. I can watch my boost grow like normal on my boost gauge and hear the turbo spooling loudly, but the power still hesitates until it finally kicks in and I get seriously launched. This continued to get worse and my Check Engine Light finally came on. I took the car in to get my final BEP Oil change and have the hesitation looked at at the same time.

CEL read a code from the knock sensor, and we cleared it. MAF sensor was dirty, so we cleaned it. Battery was nearly dead so we replaced it and gave the car a while to idle and ECU re-learn. No help at all, so we swap out the coil packs for some extras. No help at all.

Today, I take it to the subie dealership here in Vegas and they diagnose a bad MAF sensor, replace it ($$$) and send me on my way. The car is driving beautifully, no hesitation, good power. I'm finally happy for about 10 minutes. I try to drive gently from a stop light on my way home and the issue is back!!! Hesitation quickly gets worse until it's exactly how it was, and then right before I get home, the CEL comes back on.

So, my questions, if anybody would be so nice as to help me out with some answers:
1. What are some other faulty parts that might lead to severe hesitation? (Apart from MAF, coil packs)
2. What does it tell me that the car drove fine with a new MAF for only 10 minutes? As in...did another problem just break my brand new MAF, or is it more likely that I didn't even need the new MAF, and something else temporarily changed when they replaced it?
3. What could someone screw up in an engine rebuild that might cause this issue, or might cause broken parts that would then cause the issue?

I'm headed back to the dealership tomorrow, but would prefer to have some ideas ASAP so I don't keep running around spending money on new crap that doesn't fix my problem.

Many thanks to anyone who can offer any advice!
 

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Discussion Starter #3
Thanks for the quick reply.

I haven't gone into any heavy diagnosis yet. Do you mean checking for cylinder leaks or turbo system leaks? Either way, the answer is no, but I'm curious what you're thinking.

I don't have an AP. Are you thinking that my ECU is learning bad values somehow? I could theoretically test that by ECU reset and driving without learning period, right?
 

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First of all...I am by all means an amateur..
It sounded a bit like a bad boost leak from your description but I might be misunderstanding it..
turbo spools up to boost and you go nowhere or once it spools you're off?

I meant just pressure testing everything after the MAF. Any leaks after the MAF would effect your fueling and the ecu would respond. You could reset it but any action the ecu is taking is probably the best for your engine. It would be nice to pull the codes and figure out exactly what it's doing..you could try autozone or something but the dealer might be of further help. I'd start with assuming the new MAF sensor is fine, but everything is possible.

As far as what can be messed up during a rebuild (or several) just about everything is on the list. Whatever is going on could just have to do with car age/down time but the things I'd look for a dealer/shop to mess up would be banging an injector or something else fragile like a sensor...or the intercooler hoses, another gasket, or the engine work itself.
 

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Discussion Starter #5
I'm definitely an amateur, so I'll take any new insights into where to look.

To answer your question, I watch my boost gauge go up like normal, but power follows with an extremely unpredictable time lag. Sometimes it hits almost immediately with the boost, other times I'm sitting there listening to the turbo whine and seeing my gauge at full boost, but power won't kick in for 2 or 3 seconds. A leak seems strange in that case, because that would just rob me of boost entirely (can't build up psi...shouldn't matter much whether or not the leak is after the MAF, because the leak would depressurize the whole system). Also, once the power comes, it stays on...just takes its sweet time to follow what the boost gauge is telling me and what my ears are telling me.

I'll try an ECU reset and then drive a little without waiting for relearn.

A couple other details:
1. I did my timing belt and pulleys while the engine was out of the car.
2. I got adjustable cam gears installed since one was damaged during rebuild, and I'm expecting to do a turbo system overhaul at some point down the road.

One more question as well:
4. If my knock sensor is bad (or if I actually had a knock), would I expect to only see the problem after my CEL comes on, or can the ECU detect knocks and mess with boost control without throwing a CEL?

Once again, many thanks to anyone who can help.
 

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Well a partial leak could still leave you with the ability to hit peak boost, your turbo would just need to work harder to begin building boost and to maintain it. Depending on the leak it could expand/contract under so and so conditions. Fuel is metered for the volume of air passing through the MAF so even pre turbo leaks can effect drivability.

If you're actually knocking, which is likely if your knock sensor isn't faulty and you're all stock and don't have a lot of noise going on, then you might not want to reset the ECU. It will just relearn whatever it did before when it encounters the same problem.

The ECU will pull timing based on feedback from the knock sensor.. I don't think every knock from the knock sensor throws a CEL but a significant knock event would. Hopefully someone with some knowledge can chime in on that lol.

If you have any codes go pull them with a code reader from an auto parts store and look them up.
 

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^^ What he said.

Sounds like you have a leak after the MAFS.
 

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i had a similar story to this one as well, engine rebuilt drove fine for a little, then loss of power under 3500 rpms, took to dealer due to under warranty, dealer thought it was sensors (knock of MAF), nope, eventually (2 weeks) they figured out it was miss firing in cylinder 4 and the car got all new pistons along with some other small stuff, i no longer have this car and defiantly a amateur myself but figured i would throw in my 2 cents, good luck!
 

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i had a similar story to this one as well, engine rebuilt drove fine for a little, then loss of power under 3500 rpms, took to dealer due to under warranty, dealer thought it was sensors (knock of MAF), nope, eventually (2 weeks) they figured out it was miss firing in cylinder 4 and the car got all new pistons along with some other small stuff, i no longer have this car and defiantly a amateur myself but figured i would throw in my 2 cents, good luck!
Why say anything at all...Tell us what the cause was not that it was a misfire and you got rid of it. Its like your trying to make people worry.


If its hesitating bad and your getting a code for the knock sensor then maybe you should replace it. Especially since it came back. The sensor can be faulty for awhile but the fact its working is what prevents the ecu from throwing a code. Its got to have an actual break in the circuit to throw it. Explaing why so many subies get hurt from being driven with bad sensors for so long because it takes so long to throw the code. I caught my o2 sensor going bad because of my A/F ratio gauge. If it wasn't for that I would have been cruising 10.0(stupid rich) for miles and miles just dirtying up everything with carbon build up, fouling plugs, and risking detonation.

If its bad then its pulling timing to save a motor that is already fine. Thats the loss of power...Just test the sensor and make sure your getting proper resistance and voltage. That'll be the tell-tale right there my friend, real quick.
 

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Discussion Starter #10
Thanks for the replies.

I'm taking it back into the dealership tonight so they can have at it for a day or two and figure out what is going on. Their current theory is that the electrical system is shorting and killing the MAF, but it'll be quick to test the new MAF they just put in and see if it's actually dead or if my old MAF being dead was just one piece of the puzzle.


I'm seriously hoping I don't need new cylinders or anything that requires a THIRD engine rebuild, because I can't afford that kind of work again.

I will ask the dealership to check the knock sensor and whether there's an efficient way to check for a leak in the turbo system.

Will post again if we figure anything out. Seems that a lot of different issues can cause hesitation, and perhaps I can help someone down the road with info on my fix.
 

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Thanks for the replies.

I'm taking it back into the dealership tonight ... I will ask the dealership to check the knock sensor and whether there's an efficient way to check for a leak in the turbo system.
Yes checking for boost leaks is a std thing usually done early in a diagnosis.
 

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Discussion Starter #13
Update for anyone who happens to have similar hesitation issues now or in the future:

It was a combination of two different issues. First was a bad MAF. When this was replaced there was a noticeable improvement in response, but the power still hesitated after the initial response. Second, which took forever to find, was an electrical pin that had backed out of its connector and continued to back out on its own as the car vibrated and warmed up. They "fixed" it and the problem came back after 10 minutes of driving, which led to a replacement for the connector once they noticed it had backed out again. Car has been running great for a week now with more power than I remember having.

Hesitation: clean maf, test maf, swap out maf for a friend's, check various electrical system wires are conducting properly from end to end, check connectors for backed out pins...it's all cheap to try, so might start with that.
 
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