Dropped Compression Middle 2 Cylinders

[Fitzi722]

So I just bought a 1990 GSX. I knew from start that this was going to be a project, but wanted it anyway

Well we thought the problem initially was just the timing. The car was terrible to start cold, and I would need to keep revving the engine for a few minutes before the car would idle normally (if i didn't do that it would shut off).

We replaced the timing belt, tensioner, and some of the pulleys. The car wouldn't even try to start. We found a blown fuel line. Fixed it. Now the car does the same thing it did to start with... but it also backfires while revving the engine...

We decided to do a compression test. Got 145-125-125-150. So something is up with the middle cylinders. We pulled the head off today and started looking at it. (Oh btw the headgasket looked decent, no noticeable problems with it).

While taking the head off we took a look at the pistons and cylinders and everything looked good. No scratches or gouges on cylinders and the pistons have no dings, etc (which I assumed would be the case as the bottom end was replaced by the previous owner 13,000 miles ago).

We took the cams off and and checked each of the valves by spraying some carb cleaner in the ports and seeing if anything leaked out of the valves. The exhaust ports on the middle cylinders leaked a little bit, but not much. There is a **** ton of carbon buildup in both the exhaust AND the intake ports of the head. I have NEVER seen soo much carbon buildup. So I assume my problem is the head, but I expected to see a more noticeable problem with the valves or head upon inspection.

At this point I'm wondering if anyone has any advice as to how I should proceed. Any help would be appreciated.

 

[ilco187]

From past experiences a head gasket can look good but can have problems you just cant see, did you do it wet and dry to make sure of the rings.

 

[Fitzi722]

Didn't think it was the bottom end, so we only did the test dry. Looking back on it that was probably a mistake. I have to assume because of how pristine the pistons/cylinders look that it's a problem with the head, and not the bottom end. The pistons were also dry when we pulled the head, and I feel like there would have been oil on the pistons if it was the rings ( we spent two days cranking on the engine trying to get it to start/run and doing compression test).

I plan on replacing the head gasket regardless of what else I do, so that will be covered.

I guess my real question/concern is could all this carbon buildup, and what only seems to be minor leaks in the valves, cause that kind of dropped compression?

 

[ilco187]

depending on what bottom end work he had done could have been just the crank and rod bearings, or if someone did a full rebuild they could have over gapped the rings. as for all the carbon buildup im thinking a bad egr valve.

if the valve seats look good and you said they were leaking a small amount you could lap them if they are not badly pitted.

 

[Fitzi722]

I hadn't thought of the EGR Valve. I'll have to look into that.

From what I was told the short block was replaced, with what I'm not sure. I know there are 2g pistons in it now, and it is still a 6 bolt crank.

I suppose the EGR could cause those type of rough idle issues but doesn't account for the compression issues.

Maybe I should replace the EGR, really clean the **** out of the head and re-seat the valves. Worst case it still has issues and I've only wasted time and a little money.

 

[Benshooter]

You should clean and lap the valves. I've seen excessive carbon build up lead to low compression.

 

[DSMDEMON93]

+1 on lapping the valves. Have you checked your piston rings as well to make sure they are seating right? By the way, make sure you change those valve stem seals while you're here :thumbsup:

 

[Fitzi722]

Thanks for all the recommendations guys. I think my next move is going to involve replacing the head one way or another. The cam journals are ****ed and after hearing stories of abuse that the previous owner performed I think it's time to retire this head.

Now I am stuck waiting on another person for a few days. I found a local person selling a 4g63 motor out of a 90 or 91 TSI for fairly cheap and it looks good. Problem is It's been almost a week since I talked to him so I'm not sure if he still has it and he hasn't texted me back yet. Hopefully he still has it and is still willing to sell

If not I am going to get a rebuilt head with valves and springs/retainers and slap this car back together.

Like I said either way the car is getting a new head, and hopefully it will solve my problems. If not I'll be back to the drawing board .