1997 GP GT Hissing noise after turning engine off

Hi guys, I hope you're all OK!

So... I've been fixing things here and there (hub bearing, struts, etc.). My latest repair is the water pump, it was howling for a few seconds during cold starts; I just replaced it today, hopefully this unit lasts for a long time.

A few days ago, I noticed this hissing noise after I turn the engine off at normal temp. Since there was a leak from the radiator drain, I thought that was it and didn't care about it. I just installed a new o-ring on the drain cock along with the new water pump. Test drove the car and it turns out the noise is still there, I have no idea if it's normal or not, I didn't put much attention before.

The noise can be heard from underneath and above the engine, it kinda seems to be coming from the intake BUT I find it weird since I installed a new upper intake manifold in november of 2013. It's a Dorman unit and I followed all the instructions from the kit and the workshop manual. Lower intake manifold gaskets were replaced back in november 2009 after the engine was rebuilt because of a cracked head (not sure what type did the mechanic use, back then I didn't know much about repairs). It's hard to believe that these gaskets have failed already. I also replaced the coolant elbow almost two years ago after the belt tensioner assembly failed.

I'll appreciate any ideas.

Oscar.

I'm gonna guess and say that you are hearing the high and low sides of the a/c system equalizing after the car is shut off, does it quit after a minute or so? If so, this is a normal condition.

I'm gonna guess and say that you are hearing the high and low sides of the a/c system equalizing after the car is shut off, does it quit after a minute or so? If so, this is a normal condition.

Thanks for your comment!

That's interesting, it does seem to be louder close to the high pressure side (is that the one at the firewall driver side?). I'm familiar with the A/C hissing after using it but could that also happen if it's not being used?

Oscar.

If you are not using the a/c, are you running the hvac on the defroster setting? this will also run the compressor.

No HVAC at all.

I checked the level from yesterday and the radiator is full, the reservoir is at the middle and it's no longer leaking onto the ground.

Before fixing the leak at the radiator drain, the level on the reservoir would drop from 50% to 25% in around 4 weeks and the radiator was always full to the top; I think that's consistent for the size of the leak. If something else was leaking, I think I would see it dropping way faster, don't you think?

I'll keep driving it to see if the level drops. Any other ideas?

Oscar.

No HVAC at all.

I checked the level from yesterday and the radiator is full, the reservoir is at the middle and it's no longer leaking onto the ground.

Before fixing the leak at the radiator drain, the level on the reservoir would drop from 50% to 25% in around 4 weeks and the radiator was always full to the top; I think that's consistent for the size of the leak. If something else was leaking, I think I would see it dropping way faster, don't you think?

I'll keep driving it to see if the level drops. Any other ideas?

Oscar.

So you verified that your AC compressor is not running? I know you said no AC, but is that because you have it off, or you actually witnessed that it is not running?

So you verified that your AC compressor is not running? I know you said no AC, but is that because you have it off, or you actually witnessed that it is not running?

Yes, I did. I just got back home and left the engine running while I looked at the A/C compressor clutch, it's not moving at all.

I used my mechanics stethoscope to probe around and the high side for the A/C (close to the airbox) had some pounding noise coming from it, it seemed to be pounding at a rate of 2 times per second and it became slower with time. I probed directly on the upper radiator hose and I was able to hear a very slight gurgling and one or two air bubbles.

I checked the coolant level this morning and it was still at the top, no external leaks. Right now I couldn't open the radiator to check the level as it's still too hot, but will check it again tomorrow morning.

The noise seems to be either coming from or just resonating through upper the intake manifold; I'll try another radiator cap just in case, the current one is not that old, maybe a year old, but since I have a spare, I'll give it a try.

I definitely don't like the route things seem to be heading to, because it likely means I'll have to redo the upper intake after I replace the LIM gaskets. That means having to get new gaskets for everything!

As always, I'll appreciate any suggestions.

Oscar.

EDIT.- I tried taking a video of it, but my cellphone is not able to differ between the hissing and ambient noises.
I also forgot to say that the car has been sputtering for a second 90% of the time ONLY during cold starts, the RPMs sometimes go up to 1500 but come down within 1 second. I don't see any smoke while starting AND there is a little shaking on the steering wheel while idling in gear only. Spark plugs (NGK) and wires (ACDelco) have 3k on them and coil packs are originals but their static resistance values are within spec.

A quick update:

So far, the coolant level remains at the top in the radiator and the reservoir still shows no noticeable change.

I used the spare radiator cap I have and there was no change, however I noticed the noise seemed to be coming from the throttle body, this morning I decided to take a shot and check the bolts holding the UIM. It turns out they turned 180-270 degrees before the torque wrench would click at 89 inch pounds; I don't know what happened as I'm pretty sure I tightened them to spec...

I'll see what happens after I drive it for an extended period of time, hopefully that fixes it.

Oscar.

Another update:

I drove the car during last weekend after re-tightening the UIM bolts and the noise is definitely less audible than before, there are no visible leaks; the shaking while idling in gear also went down.

Since the original problem improved, I decided to give it another look (as I was short on time, I couldn't fully tighten the bolt under the alternator bracket) at the bolts, they still had a very slight (maybe 20 degrees) movement before the wrench would click (again 89 in-lbs). Since today I had lots of time, I checked and checked several times by following the torque sequence. No doubt they are uniform this time.

So far we're good; as I mentioned in the original post, I replaced the water pump and refilled with tap water (for test purposes). Today, while I was checking the coolant level, I saw this milky-brown sludge on the radiator cap and around the filler neck, I was able to get it out with my finger. I don't know what the hell that is, is it because of the tap water? Dex-cool was flushed out several years ago so I know it's not it... The old pump did have some white sludge (from calcium) inside, as I kept refilling with ta water because of the leaking drain cock. I checked the oil level and it's appearance and it's all normal, level is at the top and no milkshake color at all.

Please guys, I need some input, maybe I'm running in circles or maybe I'm giving it too much importance?. Do you think having the cooling system professionally flushed will be enough or is there something else causing the sludge?

Thanks in advance,

Oscar.

EDIT - I've upoaded 2 pics to show what came out from the radiator filler neck, that's around 90% of the total sludge I removed. The other is from what the old water pump looked like when I removed it.

I have a '98 GTP. Last year, I went to change my coolant and noticed a strawberry yogurt goo in my radiator fill neck. I panicked. Someone told me that my radiator transmission cooling heat exchanger was leaking into my radiator coolant. So I put a new radiator in it. Shortly after that, my check engine light started going on and off, indicating a lean mixture. To make a long story short, my Lower Intake gasket was leaking. I just did that repair. So, I am suspecting that the sludge is from a leak in my manifold. I don't know how that could be, but my manifold gasket was leaking. We will see if my sludge goes away. Just some info for you to think about. I can't imagine those bolts you had to tighten coming that loose.

I have a '98 GTP. Last year, I went to change my coolant and noticed a strawberry yogurt goo in my radiator fill neck. I panicked. Someone told me that my radiator transmission cooling heat exchanger was leaking into my radiator coolant. So I put a new radiator in it. Shortly after that, my check engine light started going on and off, indicating a lean mixture. To make a long story short, my Lower Intake gasket was leaking. I just did that repair. So, I am suspecting that the sludge is from a leak in my manifold. I don't know how that could be, but my manifold gasket was leaking. We will see if my sludge goes away. Just some info for you to think about. I can't imagine those bolts you had to tighten coming that loose.

Thanks!

Sometimes I like to connect my scanner while driving to see if there's anything wrong, so far I can say that all PIDs are within normal values, sometimes it starts running very rich, but it looks like that's a problem with the reman Cardone MAF sensor (value going to 2.9 g/s or less). I've read most people complain about them...

I haven't driven the car since sunday (on monday I only started it and checked for leaks), so I don't know if the sludge continues to build up or not. I'll drive it later today and see what happens. I can normally figure out problems by myself but this one has me a little stumped as I can't believe the LIM gaskets from late 2009 have already gone bad.

I also agree that's a little weird the fact that I had to re-tighten the bolts, maybe I didn't do it properly the first time, I know I followed the torque sequence and did it in 3 steps (22,44,89 in-lbs), maybe I missed something, I don't know, guess I'll find out after I drive it today.

As always, I'll appreciate any further comments.

Oscar.

Yet another update:

Car has been running fine, no overheating and no external leaks. The hissing noise is barely noticeable after turning off the engine, like you really have to try to notice it. I wasn't able to time it last night to see for how long it lasts but it doesn't seem to last long.

However, I'm still noticing the build up under the radiator cap; like I said before, oil level remains normal and hasn't dropped a bit. I'm still finding it weird that it started just after I replace the water pump. Transmission fluid is also OK since the last time I replaced it. I don't have a block tester, nor can find one, otherwise I would be all over it to find out what's wrong.

RKVONS how long was it between the day you noticed the goo in the filler neck and the replacement of the LIM gaskets? I plan to do it myself, the thing is that I can't find the aluminum gaskets around here and I'm planning to order them online and import them to México. Did that goo looked like in the pic I just attached? Let me know if your problem is gone.

Oscar.

Hello guys,

Thought I would update this thread.

I had the cooling system flushed with a professional machine and that got rid of the sludge building up at the radiator cap and filler neck.

The hissing noise was still there; it seemed more and more like bad LIM gaskets so I decided to buy new aluminum gaskets and hoped I would find something wrong with the old ones. Upon disassembly, I noticed lots of rtv over the gaskets, the bolts were unevenly tightened, some were barely tight, some were gorilla tight, the gasket on bank one was cracked at one of the cooling ports, that explains the nose being more noticeable towards the firewall. The "mechanic" that did them before thought it would be a good idea to do things with his feet instead of using his hands.

Honestly, it was a relief to see such a bad work, at least I could be sure that the new gaskets and proper work would get rid of all problems.

3 days after completing the replacement, there's no more noise and also no more shaking while idling in gear at red lights; a sputtering that would happen for a second only during the first start every day has also fine away.

Hopefully this helps anyone else with the same symptoms.

Oscar.