Need help diagnosing after rebuild.

SAATR said:

Your MAP is high for idle.

If you subtract barometric pressure from MAP you'll get manifold gauge pressure.

My engine:

5psia - 15psia = -10psig manifold pressure

Your engine:

7psia - 12.5psia = -5.5psig manifold pressure

Either your suspicions about a vacuum leak are correct and you haven't found the source, your vacuum problems are being caused by the tune or engine mechanicals, or your MAP sensor is faulty.

I would plug the brake booster line, the PCV vacuum hose, and the crankcase fresh air supply on the passenger valve cover and run the engine. You should get positive pressure out of the PCV port in the valve cover. If you get a vacuum, you have an internal vacuum leak.

Failing that, I would verify the MAP readings with a known good gauge and see if you don't have a faulty sensor.

Compression test, as suggested by Schurkey, should verify good valve and ring seal.

I'm assuming that the data you are showing is a cold start and run by the low 14.1 A/F ratio and high desired idle. Hot running numbers would be more helpful. If I'm wrong and that IS hot idle, I'd like to know what the target A/F ratio is.

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L31MaxExpress said:

Was this a stock rebuild or does it have a bigger cam? Was the cam degreed when installed? I have seen guys miss the cam install by a full tooth and sometimes two. Even 1 tooth off will make them run terribly. If the cam is retarded from the desired centerline, idle vacuum and low speed power will be low.

I accidentally retarded a cam in a 305 once. 1 full tooth off on the timing set. It struggled to idle, was hard to start and ran terribly under ~3,500 rpm. From 3,500 to 6,000 rpm it pulled like a circle track engine. I knew something was up because the little RV cam should have run out of steam at 4,500-5,000 rpm. Cranking compression test verified the cam timing was in fact off. The engine had cranked previously at 185 PSI. It struggled to make 140 psi. Since then I have always degreed cams, even stock ones when install them. Spending $150 in stuff to do it and an hour to degree the cam beats having to tear the whole front end down again and fight the oil pan in chassis. The investment in degree stuff was worth it to me because I have built 10-15 engines since then with it.

Valve adjustment that is too tight can also make a SBC a complete dog. Even the FSM spec of zero lash plus 1 turn is far too tight. I do a running adjustment to find zero lash and then 1/4 turn.

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