Guys with GTP-6, GM847 and larger cams inside!

I just want to know if you guys have problems with not enough vaccum at idle and it throwing the MAP sensor out of wack. If so what did you do to correct it? Thanks.

Randy

No vacuum problems with the GTP6 and an M6. Its on a 114 tho...

Drives and sounds WAY tamer than the specs.
Ryan

Mine drives great, but doesn't sound very tame:D No issues with brakes and it's an auto.

Well, I haven't checked my vac with a gauge, but the MAP is reading 4.01 volts at idle and 4.91 volts is WOT. Just to let you know, my specs are 235/242 @ 0.050 .555/570 with 1.6's on a 112 LSA. I really don't know what to do I've checked the MAP and it's OK, I installed a new opti, but I still get a DTC 33.

Randy

I'm having the same problem, but I'm running a big cam on a 355ci. Here's my specs. on the cam 242/252 @ .050 .525/.540 with 1.6's on a 112 LSA.

I only get the DTC 33 (service engine light), when I'm at idle while in drive, but when I'm in park or neutral at idle the service engine light disappears.

Have you tried to spray carb cleaner around the intake and vacuum hoses to make sure you don't have a vacuum leak?

Jody J.

I used to get the 33 code every now and then, but never had any problems with the brakes. Had the car retuned, and idled up to around 1100 or so and hasnt thrown the code since.

I have no issues on my GM 847 in my 383. I'm running 11:1 static compression and it seems to run without a hitch . I have it idling at 950rpm fine, lobes and shakes a little ther so I may jump it up to 1000 rpm, my brakes work fine as well. I dont have the data handy but my map was reading in the 4.1 range as well.
My problem is the rv limiter and traction, I have to move my rev limiter up to at least 6500 , its set at 6 now and I bang off the limiter on every gear at WOT...lol

I called Joe Overton and we discussed my problem and he thinks it mostly in the tune. He also told me to readjust my valves with 1/8 of a turn of preload and just for good measure I'm going to replace the MAP sensor. Once I do this I'll check the vac and see if an increase in idle speed is necessary.

Randy

Originally posted by rje
I called Joe Overton and we discussed my problem and he thinks it mostly in the tune. He also told me to readjust my valves with 1/8 of a turn of preload and just for good measure I'm going to replace the MAP sensor. Once I do this I'll check the vac and see if an increase in idle speed is necessary.

Randy

Hey Randy,

Thanks for sharing the info. I'll have to try and re-adjust the rockers. I've got mine set to 1/4 pass zero lash.

I was also browsing Christian Millard's web pages and stumble across this page regarding timing and vacuum. http://carprogrammer.com/Z28/PCM/Ignition/OptimizeSparkAdvance.html

Jody J.

OK, I checked the vac at the fp regulator and at idle, 850 rpms, it has just over 5 in/hg. If I increase the rpms to around 2500-3000 rpms the vac is 14in/hg and the engine runs much smoother and has much better throttle response. I truly believe that my problem is in the tune, and this is what I think will cure it, so tell me what you think. First, I plan on raising the idle rpm to 1100 rpms, take some fuel out, and add some timing, from 25 to 32, and see what that does.

Randy

I'm running a 230/245 .622/.640 XE cam on a 113 LSA. Getting 14" of vacuum at 950rpm idle. Preload is 1/16th past zero on Comp R's.

I'm hoping for that when I get everything in order. You obviously have a better tune than I, and those extra 43 cubes don't hurt either.

Randy

Randy,

Did you use an analog type vacuum gauge to check vacuum? If so, the needle should be fairly steady at any RPM, even idle. If it isn't, if it bounces around a little at idle and gets more steady at higher RPM, you probably have a problem other than tuning. Could be a vacuum leak, weak or broken valve spring, lifters/rocker arms adjusted too tight, etc. Can't think of any way tuning could affect vacuum at idle that much. 5 in. Hg. is way too low even for that cam.

Mark, man you just don't know how much searching I have been doing to find a problem other than tuning, but to no avail. I used analog gauge and it was steady at 5.5in/hg @ 850rpms and it was also steady at 2700rpms with 14in/hg, so no vaccum leak. The valve springs were just checked and the valves have been readjusted twice to a 1/4 turn of preload, so valve train is fine. Now to the tuning, from all the info I'm read and to all the tuners that I've talked to I really believe that I need a little higher idle rpm, 1000-1100, more timing than 25 degs, more like 34-35 degs, and possiblly a little fuel taken out. We will see, I plan on making those changes this weekend. Later Mark.

Randy

On a side note I have noticed on my breaks, they work great, but my ABS seems to kick in more, could that be because lack of vac?

Originally posted by rje
Mark, man you just don't know how much searching I have been doing to find a problem other than tuning, but to no avail. I used analog gauge and it was steady at 5.5in/hg @ 850rpms and it was also steady at 2700rpms with 14in/hg, so no vaccum leak. The valve springs were just checked and the valves have been readjusted twice to a 1/4 turn of preload, so valve train is fine. Now to the tuning, from all the info I'm read and to all the tuners that I've talked to I really believe that I need a little higher idle rpm, 1000-1100, more timing than 25 degs, more like 34-35 degs, and possiblly a little fuel taken out. We will see, I plan on making those changes this weekend. Later Mark.

Randy

Sounds like you've been covering all the bases and pulling your hair out ;)

Still, how would tuning affect vacuum? Manifold vacuum is created by the valves opening and closing and the up and down movement of the piston. Raising the idle will improve it, but it sounds like a band-aid fix for a mechanical problem. Changing the timing won't affect vacuum significantly. Adjusting the fuel rich or lean won't change that. If it was a little low I'd say it's the bigger cam, but 5 in/hg at 850 RPM is too low, IMO.

May you have some lifters that aren't staying pumped up at idle?Just trying to think of some things that may be causing it :confused:

I appreciate the help Mark. Every tuner that I've talked to, except the one that tuned my PCM, said that I need more base timing to increase the cyl pressure and inturn increase vac. Also the rich condition that I have is just compounding the problem because as one tuner put it, " excess fuel is anti-vaccum". Just as an example I talked to to a guy who was running some insanely big cam that had a 104 intake centerline with 4degs of adv ground into the cam. He ran 44 degs of timing at idle and made 12in/hg vac, that's crazy. So if a cam of that size can have vac, then I should have no problem whatsoever in increasing my vac, but we will see this weekend if it's tuning or if I'm just a total moron that can't find a vac leak.

Randy

Originally posted by rje
I appreciate the help Mark. Every tuner that I've talked to, except the one that tuned my PCM, said that I need more base timing to increase the cyl pressure and inturn increase vac. Also the rich condition that I have is just compounding the problem because as one tuner put it, " excess fuel is anti-vaccum". Just as an example I talked to to a guy who was running some insanely big cam that had a 104 intake centerline with 4degs of adv ground into the cam. He ran 44 degs of timing at idle and made 12in/hg vac, that's crazy. So if a cam of that size can have vac, then I should have no problem whatsoever in increasing my vac, but we will see this weekend if it's tuning or if I'm just a total moron that can't find a vac leak.

Randy

Hey Randy,

I'm certainly no expert on these EFI engines, but I used to be pretty good on carbureted engines in my time ;) Some of these things just don't make any sense from a functional mechanical standpoint.

I can understand some of these things to a point. But like I said, not to the point of having only 5 in/hg at 850. Seems like you would have to just about flood the cylinders to loose that much vacuum. In which case the engine wouldn't run :confused:

Ignition timing should have little effect on manifold vacuum. But cam timing is another story; it dictates vacuum.

Have you physically checked the cam to make sure it's what it's supposed to be? Maybe they miss-marked it and it's not what it's supposed to be. Or maybe it just wasn't ground right in the first place.

An intake manifold leak can be on the underside of the intake too. Sometimes the only way to find it is to pull the intake and look at the gaskets.

I'm very interested to see what you learn and find out. Good luck with it!

Mark

Thanks Mark, I'll keep you posted on what I find.

Randy

Originally posted by Raptor1
Hey Randy,

I'm certainly no expert on these EFI engines, but I used to be pretty good on carbureted engines in my time ;) Some of these things just don't make any sense from a functional mechanical standpoint.

I can understand some of these things to a point. But like I said, not to the point of having only 5 in/hg at 850. Seems like you would have to just about flood the cylinders to loose that much vacuum. In which case the engine wouldn't run :confused:

Ignition timing should have little effect on manifold vacuum. But cam timing is another story; it dictates vacuum.

Have you physically checked the cam to make sure it's what it's supposed to be? Maybe they miss-marked it and it's not what it's supposed to be. Or maybe it just wasn't ground right in the first place.

An intake manifold leak can be on the underside of the intake too. Sometimes the only way to find it is to pull the intake and look at the gaskets.

I'm very interested to see what you learn and find out. Good luck with it!

Mark

Well said. When i initially installed my cam i had the same issues as what was said above.

Ended up tearing down to the timing cover again, and sure enough, my dots werent lined up, i missed a tooth :(

I found the problem! The can was ground wrong, got a new one of the right specs and it's money. It holds 13in/hg vac at idle and I plan on running it tonight to see what it will do.

Randy

Originally posted by rje
I found the problem! The can was ground wrong, got a new one of the right specs and it's money. It holds 13in/hg vac at idle and I plan on running it tonight to see what it will do.

Randy

Hey man, that's fantastic! Not that the cam was ground wrong, but that you found the problem. ;)

Let me know how that sucker runs! :D

As soon as I threw heads on my CC306, it started throw MAP codes. But it also turned out my IAC was maxing out at idle because my throttle body blades were closed too tight. I adjusted the throttle stop screw and both problems went away.