I'm just about at the end of my rope. I just spent 3 hours at the shop trying to figure out what the hell is wrong with my motor. I had it rebuilt in December and it has been down on power ever since. It's an '86 with iron heads, which were ported at the time of the rebuild. I would have gone aftermarket, but money was getting tight as all the little crap kept eating away my budget. I used a TPIS 700-135 camshaft (280 adv dur 224@.050 .495 lift) So when the motor goes back in the car, we fired it up and it sounded really good. When I took it out to enjoy all my new power, my heart stopped. It wouldn't even spin the tires. But under wide open throttle on the 1-2 shift(auto by the way)the car would go a little sideways. It even broke loose on the 2-3 shift which it NEVER did before. So it had power, just nowhere near the rpm's I wanted it. Other problems included terrible gas mileage, runs about 10 deg hotter than it used to, and worst of all I only have 10 inches of manifold vacuum. So I called TPIS and they said it should run like a scalded ape right off the line. They said it sounded like the cam sprocket was off a tooth. They also said the vacuum reading didn't sound right. It should be pulling 16-18 inches. So today I bring it back to the shop to start checking things out. I figure cam timing is off so we pull the timing cover off. The dots are lined up perfectly, but I know that doesn't necessarily mean anything. The timing mark on the cam itself could be off. Then we thought the rings could be undersized so we checked into that. Leakdown test passed with flying colors and a compression test checked out between 125-135 across all 8 cylinders. Could be a little higher, but doesn't explain the problems I'm having. I keep coming back to the cam. A MAF car should be able to handle durations up to about 230@.050 without other mods, which I am under. I've also noticed that many forum members are running bigger cams with no problems. I ouldn't mind switching to a smaller cam, but I'm not convinced that's the problem. Everyone I talk to says that cam should run great in this car. I know I need a new computer chip, but that wouldn't give me any vacuum, would it? Does anyone have any ideas on what I should do next? If I need to switch cams, is anyone running a smaller Comp Cam (252 or 256) and are you happy with it? Thanks in advance for any suggestions.

 

It would be helpful to know what all was done to your engine during the rebuild. I'm seeing cam change and porting the heads. The thing that bothers me the most about the info you provided is the compression numbers. You're not going to make a lot of power with numbers that low. They need to be in at least in the 150 to 160 range. A lot of things can affect that though, like cranking speed during the test. Were all the spark plugs out, or just one at a time? And was someone holding the throttle open during cranking?

You can increase the compression psi with a higher CR of course, but you can also increase it by advancing the cam timing. Did you degree the cam? And if so, what is the intake lobe center set at? If the cam gear was off a tooth it would be running a lot worse than it is now, if at all, but given all the tolerances, your cam could be retarded 2-4 degrees, and advancing it 4 degrees or so (more if it's currently retarded) will pep it up a lot.

Hope that helps.
Dan

 

I don't know how far you went but did you match the springs for the cam? It sounds like a mild cam you have. What about 1.6 rockers? You also have to look into a mod computer and exhaust. A cam is like a heart if everything doesn't match it then you can't inhale or exhale right. :thumbs:

You should specify other mods for better response. :thumbs:

 

Wow! That IS a tough one. The first thing that came to my mind, was off a tooth on the timing chain. I don't have experience with this, and maybe Danno 85, is correct that it wouldn't run that good, off a tooth. My next thought was ignition timing. While it could explain how it runs, it doesn't address the compression reading so low. My worn out, stock, oil burning (a quqrt in 200 miles or less) Crossfire has 165# compression in the lowest reading cylinders. A BIG cam will lower cranking compression pressures, but you don't have a big cam. Could lifter preload be WAAAY to much? I'm betting it's something fairly simple. The problem is in the diagnosis. I hope some of these thoughts get you headed in the right direction.Good luck, and...

RACE ON!!!


[Modified by CFI-EFI, 6:52 PM 3/20/2003]

 

Yes the springs are matched for the cam. The 10 inches of vacuum is the biggest indication of a problem. Intake or exhaust mods will not effect that. Dan, you're right about compression numbers. They need to be higher. The cam wasn't degreed, the builder just lined up the dots. By the way, the shop I'm using now is NOT the same one that built the motor. Anyway, right now I'm leaning towards ring problems, but I can't afford to have the motor torn down on a hunch. If not the rings, then I'm back to the freakin' cam. Can the index mark on a camshaft be THAT far off?

 

Pressure was checked while cranking with one plug out at a time. I must confess I don't know what lifter pre-load is. We re-adjusted rocker arms today(tightened until there was drag, then 3/4 turn more) They were all spot on anyway, just made me feel like we were accomplishing something.

As far as cylinder compression, what can cause it to be low? Blown head gasket, rings, valve sealing problems, valve timing. Am I forgetting anything? Gaskets are fine, as are the valves. The vacuum gauge held steady. If it bounced around it would indicate I had a problem with a valve.


[Modified by flyersfan1088, 8:10 PM 3/20/2003]

 

You are correct in your list of causes of low compression, BUT
The leakdown test rules out the rings (and valves). You might try adjusting the valves with the engine running. It can be messy, but it is more fool proof. With only one valve cover off at a time, let the engine idle. Back off each rocker adjusting nut, one at a time, until you hear the valve "clicking". Tighten it down until it just quiets. This is basically, zero lash. Tighten the nut to your desired pre load, but no more than 1 turn more. Personally, I like a 1/4 turn. Do them 1 at a time. Shut the engine off, reinstall the vc, and do the other side. That should eliminate the valve adjustment as a possible cause.

You might redo the compression test with all the plugs removed. Make sure the TB is wide open, and the battery fully charged.

Another REMOTE possibility, is a severly clogged exhaust system, but that is pretty "far out".

You're getting closer. You'll have it solved in no time.

RACE ON!!!

 

Almost forgot something that seemed a little odd about the leakdown test. We first put 175psi of air straight into the spark plug hole. It sounded like waaay too much air was coming out of front of oil pan. The mechanic thought we found the problem. So then we hook up the actual tool and it showed down in the green for O.K.(don't remember the actual number) But the tool said to only use 75psi, which doesn't sound like enough pressure to me for a good test. The 175 is a standard procedure that this mechanic uses and he said he had never heard that much air escape from the pan before. So with his test the car failed, but using the proper tool the car passed. Go figure. By this time it was 5:00 so we called it quits until morning. When I was leaving his parting words were "I grew up with 10W-40 in my baby bottle but I've never seen anything like this" If I had a dollar for every time I've heard variations of that concerning this car, I'd spend it all on a new crate motor and be done with it :D

 

175psi is a lot, i'm used to doing leak-down tests with ~100psi. you can hear the hiss even from 3% leakage at 100psi, albeit it's not deafening. but i have no idea what the leakage would sound like at 175psi. however, if the tool was used properly, i'd trust that more than the sound.

all that said, it still sounds like the cam timing is off. it's certainly not unheard of to have a cam that is ground 20 degrees off. you need to use a dial gauge on one of the rockers (right at the pushrod) to see that the .050" events come at the right crank angle.

the other thing, of course, is ignition timing and/or ecu ignition advance control. this could affect vacuum while running, but not cranking compression tests.

lastly, any dyno runs? how far down on power is it? after all, a stock '86 setup with just ported heads and a cam isn't going to rip your head off...

 

[QUOTE
lastly, any dyno runs? how far down on power is it? after all, a stock '86 setup with just ported heads and a cam isn't going to rip your head off...
[/QUOTE]

No I haven't been to the dyno. I think it would be a waste of money right now. Hell I can't even spin the tires! I don't need a dyno sheet to show I'm down on power :D

Well I'm off to the shop. I'm going to try to track down a degree wheel to verify the cam was ground right. Wish me luck guys.

 

good luck flyersfan1088. If the cam is okay, I have a suggestion. Check to see if all the coolant flow holes to/from the head are open, and not plugged with sealant. I had a Z28 that got a rebuild and the jokers plugged the hell out of them (well, not all of them), and I experienced the same symptoms you are. :seeya

 

Just got off the phone with TPIS. He said to check knock counts. I don't know why I didn't think of that. He also agreed that I HAVE to get a new PROM burned. I know that it will help, but I'm still confused over the low vacuum readings :banghead:

 

By chance are your IAC counts going over 150? Once you go over 100 at idle, you start having a large drop in vacuum. I've had problems not related to the IAC at all, but the IAC will go full open. It will kill vacuum...which is exactly what it sounds like it is doing.

Adjust your minimum air rate screw, your timing, and if you have an adjustable FPR, adjust it to less pressure. It might end up not being a mechanical problem but just a tuning problem. Have a scanner??? What are the BLMs doing also? Good luck, -85Y

 

I'm a complete beginner at Corvette engines, and was reading this thread to start pickingup some knowledge.......but........

One of the replies here rang a bell - I just had a catalytic convertor 'meltdown' - it fused solid. (on a Fairlane) The engine would start and tick over fine but there was no power under load, it came on over a week or so. Eventually I could only just crawl away from traffic lights on the flat, any sort of hill was a nightmare.

It took the mechanics 5 days to find the problem.

So I'm just wondering if your exhaust is partially blocked, it would certainly explain some of the results.

Ok, back to the experts, and good luck, we all hate these 'mysteries'.

S'nut

 

You know it's very ironic that you posted this today. I just picked up the car(left it over the weekend so they could put everything back together again) The last thing the mechanic said before I left was that he heard one of the cats rattling. That might have something to do with the low vacuum. I personally didn't hear it over my defective whining torque converter. But I'll definitely check it out.

 

 

Oops! Sorry. Maybe not that remote after all. I'll find out in a few days. I'm going to park it until I can get motivated to look at it again.

 

Did you disconnect the EST wire when you set the timing? What do you have the timing set at.

Are you sure all the spark plugs wires are on in the correct order? It is easy to cross two of them and the car will still run.

 

Yes, 6 deg, and yes.

 

Not sure what is all been suggested already, haven't read the whole thread.

But, after my 1st round of modifications in my 85, I too basically had the same problem.... up above 3500 rpm, ran pretty good, but down low was very sluggish, also it seemed 1/2 throttle was as good as full throttle.

Advance the hell out of the ignition, 6 degree initial and my motor did not run well down low.... I literally ran 12 degrees advance. I also went to an aftermarket chip which helped quite a bit too. Make sure that TPS sensor is set correctly too.... somewhere around .58 volts idle and 4.6-4.8 wot.

One last thing, I had a converter rated at right around 2200 rpm and flashed up to around 2600 rpm or so, you should consider a $ 300 12" 2500 rpm converter.... regardless of everything else, that cam with 224 duration moved up your peaks 200-600 rpm in the rpm range.

Advance that ignition !!!

good luck
Beach Bum