Low Vac and Poor Idle With New Engine

[HardcoreZ28]

Ok guys give me some ideas why my new engine is running like dogmeat. It's an all new 383 with my Super Ram and AFR 195 heads. Cam is p/n 08-468-08 242/248 .549/.544 113 lsa all at .050 from Comp.
I started it up today and set the timing to 6* which is stock. The idle is all over the place on startup and once it warms up it stabilizes a little. I put a vac gauge on it and I'm only pulling between 9-12. I don't know if that's due to the large cam or if I have a leak somewhere. I went all around the Super Ram with some carb cleaner and later some propane and didn't get any surges so I don't think it's a vac leak there. The guys at Competition Engines (Tony Feil's old shop) built the engine and set the valve lash on the stand. I'm wondering if they set one too tight? Of course getting my valve covers off is about an hour job and I always manage to scratch the hell out of them so I won't want to go that route if I don't have to.
Only other thing I can think of is that the chip needs to be tuned more.

What do you guys think? Sound like a vac leak or tuning issue to you?

[The Fixer]

Hey Paul, what chip are you running? Is Prevost still doing your tuning? My ZZ4-cammed TPI 355 does the same thing; crap idle when it's first started, and eventually the ECM compensates and raises the idle speed to get the thing to run better. I'd bet there's something wrong with the IAC settings; the new motor probably doesn't need as much as the stock tables call for since the new cam/head combo is sucking in a lot more air at idle.

[HardcoreZ28]

I'm running a chip from PCM For Less actually. I'm gonna try to datalog tomorrow or next week if I can get my laptop to talk to the ECM and send Alvin some data and see what he makes of it.

[Ian]

is the cam any different than the one you were running before? I'm assuming it is. I would guess its tuning related. a vac leak would probably fluctuate more than just 3 in. I would still pull the valve covers and check all the rockers though. one hour of work is less than data logging, sending the file and waiting for a reply.

[HardcoreZ28]

Yes the cam I used to have was 219/219....the new one is 242/248. Prevost is gonna datalog it with me wednesday hopefully and we'll see what we find. As for pulling the valve covers....it's actually about a 3 hour job because I'd have to pull the alternator and unbolt the headers to get them out without scratching them up bad. So I'd rather not go that route.

[HardcoreZ28]

You know after thinking about it last night I'm starting to think maybe my MAF ate it. It was sitting somewhere in the garage where it may have gotten wet. Any thoughts on this one?

I'm gonna try picking one up at the local parts store tonight and throwing it in there to see what happens. I can return it if it doesn't.

I've ruled out a vac leak at this point after some testing.

[The Fixer]

You know after thinking about it last night I'm starting to think maybe my MAF ate it. It was sitting somewhere in the garage where it may have gotten wet. Any thoughts on this one?

I'm gonna try picking one up at the local parts store tonight and throwing it in there to see what happens. I can return it if it doesn't.

I've ruled out a vac leak at this point after some testing.

Are you using a de-screened MAF? I remember when Jon tuned my 305 he bumped up all the MAF tables to compensate for that. Try sticking a stock MAF in and see what happens? If it works for you, I'll go replace mine too. :)

[HardcoreZ28]

My MAF is stock. I used to have a descreened one and it gave me problems.

[Pampered-Z]

You need a different tune and probably a slightly higher idle with that cam. Plus if you have a stock converter it could be stalling the motor when the idle surges.

You’re definitely going to loose vac. with that cam. I installed two GM847 cam in LT1s: 234/242 539/558 and the brake peddle is notable softer. It is allot more noticeable on the car with the stock converter compared to the car with the 2800 stall.

Also double check your TPS. If you had to open the throttle blades to get the car to idle this changed the voltage. You might need to elongate the holes to turn the TPS to get the voltage where it needs to be at idle. This is almost a must if you run a 58MM TB.

[HardcoreZ28]

You need a different tune and probably a slightly higher idle with that cam. Plus if you have a stock converter it could be stalling the motor when the idle surges.

You’re definitely going to loose vac. with that cam. I installed two GM847 cam in LT1s: 234/242 539/558 and the brake peddle is notable softer. It is allot more noticeable on the car with the stock converter compared to the car with the 2800 stall.

Also double check your TPS. If you had to open the throttle blades to get the car to idle this changed the voltage. You might need to elongate the holes to turn the TPS to get the voltage where it needs to be at idle. This is almost a must if you run a 58MM TB.


Thanks for the ideas. The tune is probably part of my issue. I've turned the idle up slightly and it's around 1,000.....shouldn't need to be much higher than that.
Converter is a 3500 stall speed so it's not stalling at idle. I also reset the TPS and fuel pressure saturday night. I'm gonna try the new MAF tonight for the hell of it. If that doesn't work I'll be datalogging on wednesday night hopefully.

[Batman]

a 242/248 cam can certainly cause a drop in vacuum. I think the rest is tuning related but with a cam that size you are going to have to deal with some low RPM surging and it is going to idle rough.

[HardcoreZ28]

Yeah I understand the low vac reading but the surging is really bad and the thing also doesn't seem to want to pull above 3500-3800 although I haven't opened it up WOT yet. Gonna try a new MAF in an hour or so and I'll repost.

[HardcoreZ28]

No help from a new MAF....looks like I'll be datalogging on wednesday night.