I needed to replace the cam in my 84 crossfire and did with one that has a bit of that rough idle. Its not a race cam but somewhere between. Streetable but more modern.

My question is with the idle not having a perfectly smooth idle, will the differences or pulses of a slightly changing vacuum compared to a completely stock, smooth idle effect the map sensor and its signal for mixture? If so, is there some way to buffer that signal at idle? Can something be put between the sensor and the intake to help with the harder pulses at idle but not effect the driving range?

Thanks

Russ

 

If you put in a more radical cam, you now have a lower vacuum signal which the map sensor will detect. Unless you get a tune, your car won't run right.

 

A tune, I wasn't sure just what there was to tune other than the plugs, wires, cap, rotor and timing. I did rebuild each throttle body. I haven't replaced the o2 sensor and I don't have a cat converter on it yet but that is downstream. I do need to check the IAC's but other than those items, what else would you suggest?

Well I guess there is fuel pressure too, it does sound and feel to be running rich but that is right after starting it. I have not driven it yet as it isn't road worthy so maybe it will take that to get the computer to adjust to the new cam and mixture settings.

I really appreciate the reply.

Russ

 

YOU ARE CORRECT....You can not expect the engine to idle correctly with NO catalytic, possible O2 sensor issues and the FACT that the IAC's need to be reset by DRIVING it.

I would also see how long it takes the computer to go into 'CLOSED LOOP' after it has been cranked...and see if it goes back into 'OPEN LOOP'....which is NOT good.

AS for the MAP values...I would look at them after you drive it and see what the scanner shows for the voltage reading. I know what the values should be for the MAP...but I can not tell you because I talked with Gordon Killebrew to get specific information...and it is not fair for me to pay for it and just give it away. That would effect his tech line. What others will post in regards to this...is up to them

You CAN NOT GUESS that you have correct fuel pressure. This is BIG MISTAKE if you assume it is fine. And this is no picnic in checking...takes a unique set-up as the manual shows...but you can make one. TRUST ME...I check them and have found many times the fuel pressure was seriously low. So do not THINK that looking at the fuel spray you can tell...because you can not...and I have worked on well over a dozen Cross-fire engine cars before...and I cant tell the difference.


Then comes the correct procedure of balancing the throttle bodies WHAT did you actually do to each throttle body??? If the throttle shafts have wear in them (which they more than likely do). And you did not actually re-bush these shafts which requires machining....because GM never had a serviceable bushing in this location. I would send them out. BECAUSE...if these shafts have a lot of play in them...air can be sucked into the engine AFTER your IAC'S and cause for idle issues....depending on the severity of the vacuum leak(s)

Get it all assembled and sealed up. The see what happens when you drive it and look at the scanner.

DUB

 

Thanks dub. You are right about those items I didn't check or have values for. I can make a fuel pressure gauge easily but I did not get the throttle shafts re-bushed and there is play. I should also get a vacuum read at idle to see how it compares to stock but it is going to be lower than stock. It does fire right up at least but it is not right.

I will correct the points you made and let you know how it goes. Thanks for that reply.

Russ

 

Russ,

Often times when I have to work on the Cross-Fire engines and the Tuned-Port engines...they have been touched before. And I have luckily run into the Cross-fire engine top plate for the intake NOT being brutalized with a 'brisket pad to remove old gasket material from some previous SLACK MECHANIC. On the Tuned port engines...I run into it a lot and with the surfaces being damaged...I can get vacuum leaks when done and have to go back and do it again and repair the tubed runners for the Tuned port engines.

Hopefully you were careful and diligent in making sure when you torqued the bolts for the top pale of the intake you did it to GM specs. And that the cleaning of these surfaces were done carefully. And as you know...the top plate gasket is not that thick..and that is when I use one of my super thin feeler gauges and check the top plate to the lower intake when I just set it down on the intake.

DUB

 

I didn't mean a tune up but rather a tune to change the stock engine fuel management program. The 84 uses a speed density system. That means that the computer is preprogrammed to run the motor using certain perameters; engine temp, throttle position, manifold vacuum, 02 sensor readings. When you put in your more radical cam, the map sensor reads a lower vacuum. That tells the computer that the engine is in a load situation which in actuality it is not. The computer will dump in fuel and the motor will idle and run like crap. A tuner will be able adjust for the camshaft. Playing with fuel pressures won't get the job done.

 

Well I am in the process of re bushing the throttle shafts. They are both worn some and since I don't have a local place to fix them, I ordered a kit with reamer and will do that job myself. My problem is finding the screws that hold the throttle plate to the shaft. They are very small and I had to grind the tips off to get them out. I could use locktite if nothing is available but don't really want to take that chance.....but, if I have no choice...

So on an 84, there is a way to tell the computer to adjust for the cam? I have heard of chips but not anything about tuning one. What kind of tuning are you talking about. I used a thin layer of grey silicone seal with the gasket to get the best seal I could. I will test it with carb cleaner to see if there are any leaks when I get the throttle bodies done. I will make sure about the fuel pressure anyway though. There is an adjustable feature on the regulator installed by an owner at some time and I did rebuild them and I did install a new fuel pump.

One question I do have about the air pump and the tube running to the cat converter. I see where some block off the EGR valve and others that don't run the air pump. I have a working air pump and a working egr valve but should I go all the way with a cat that has the pipe feeder and hook up the air pump? Just curious.

Thanks

Russ

 

Put the air pump tubing and 'whatever' as it should be for now like the factory intended. You have 'bigger fish to fry' and I would wait until you get some issues repaired and then see what is happening.

Do you have a factory service manual???? IMPORTANT QUESTION.

As mentioned by '383vett'. DEPENDING on the cam you put in...having the computer re-programmed is questionable. If information here on the Forum or on the web cannot answer your questions. Possibly call Gordon Killebrew and he might be able to tell you if the cam specs will require a modification to its programming....or if he has a 'trick' to get it working.

DUB

 

What are the specs for the cam? TECHNICALLY, any cam change "requires" re=tuning of the chip/ECM, but in reality, you can get away with a lot using other methods and it can be "good enough". Cam specs?

 

I do have a cd version of an 84 shop manual. The cam is a Bullet cam,

duration at .050 in 237 ex 237
lobe lift .3000 both in and ex
separation 112
duration at .006 279 and 279
gross valve lift .450 and .450
hydraulic
degree intake lobe to 106

I actually retarded the cam 2 degrees for a little more upper end.

So that's about it for now. I have the bushings for the throttle bodies coming and should be here soon. The back body has an adjustable pressure piece so as soon as I fix the throttle shaft bushings I will set up a fuel pressure gauge and get see where it is.

Russ

 

That is a ton of duration on a 350 CID, speed density car w/no tuning. People who know me would know that I am the first to advocate "feeding it what it wants" mechanically, as it's free, and you can often get a large percent of the gains of tuning the ECM, for no cost. But in this case, I think you've got way too much cam. The engine IS still a 350, right? If it is, I don't think you'll get what *I* would consider to be acceptable performance, w/o an ECM (chip) tune.

Outside of a proper tune, the best that you can do, based on my experience, is get a vacuum referenced fuel pressure regulator housing meant for a marine application GM TBI; P/N 17113186. Install it on your rear TBI and adjust the pressure to ~15 pounds w/no vacuum to it and you'll end up w/about 12ish PSI at idle (if you're lucky, with your vacuum) which will help idle performance and cruising economy. You can change springs and injector sizes to affect the fueling "range" you get, on and off vacuum, so to speak. With stock compression (9:1) and that much cam, you need to move your base timing way up. I'd start at 12* BTC and play w/it from there. You MAY be able to get it to run "OK" with those methods...but if I were you, I'd start planning on an ECM swap to something easily tunable, and start acquiring tuning tools.

 

I appreciate your knowledge about this. I will work on at least getting it running so the iac's can adjust and hopefully with the throttle shafts not sucking in air I can get it driveable. If it can be made better, with your help I feel I can get it close. I will have to ask about an ecm swap later. I have the interior to finish then a repaint.

Thanks again for your help and advice.

Russ

 

Ok, I finally sorted some serious items. First fuel pressure is 10.5, within tolerances. The TPS was malfunctioning. I could never get it to the correct voltage with the throttle closed. Got a new one and got it set. I rebushed the throttle shaft and got the idle screws set. I also had a bad MAP sensor. When I pulled a vacuum on the one that was in the car it was not holding. I had another and checked it and it does hold a vacuum. I also checked and reset the IAC's. I replaced the O2 sensor too.

I replaced the plugs, checked everything and started it. It was pretty rough at first, had to hold it at half throttle to keep it running. It gradually got better and better to where I could let it idle. I let it warm up until the fan came on. The idle settled at 1200 rpms. Kinda high but that will get better when I get to drive the car to let the computer set the IAC's. The exhaust cleared out and I had a smile on my face. It is in the 30's here but my shop is in the 50's.

There are still more things to do to get it on the road, like a front seal leak on the tranny, sigh, interior, tires, sanding and painting and so on.

Thanks

Russ

 

I was catching up on some reading, and found this thread. I also have a high idle (1200 rpm), and was wondering how your rebushed TBs worked out.