Experiencing a new problem. Well, not new, but it happens on a more regular basis since I have changed cams and intake (I think as a result of more incoming air or something).

First off let me give you the rundown on what I have. Its a 66 GMC Truck with an 86 Corvette TPI in it. I have Trickflow 195 heads, Bigmouth intake manifold, TPIS 224/224 - .480/.480 cam with 1.6 rockers. It has no EGR or smog. Timing is set at 6. TPS is at .56. Th-350 tranny with 4.10 gears.

Under heavy acceleration, it will sometimes give me what I think is a lean pop. It will start to pull hard, then I will lose power and throttle response, and then sometimes it will give me a pop.

Today while pulling onto the highway, I gave it quite a bit of throttle. It took off, and as soon as I hit 2nd gear, it started to lose power. I gave it a tad more throttle and could hear the air whistling in, but it didn't give it any more gas or increase my RPM or speed at all.

Its almost as it my MAF isn't reading the air for a few seconds, or my TPS isn't gaining voltage. I used my scanner and ran my TPS through its range and didn't seem to have any sticky spots.

Any ideas? Any more info needed? Anyone had this problem lol?

Thanks in advance people.

 

Hm... I assume you are not getting any codes?

This is interesting. I wonder if there is a tear in the intake tube.

Are you sure the timing is correct? That sounds much like an over advanced timing issue.

I am wondering if there isn't an ingition related failure. Inside the distributor is a module, I wonder if it's giving up... or the distributor itself is out of functional range. Or the coil isn't able to keep up at high rpm.

Finally, bad wires will do this, too.

Just a few thoughts.

 

Check for spark and fuel to each cylinder, my bet is an ignition problem.

I suppose a clogged cat could cause it, but it shouldnt in quite this way.

 

It DOES sound like a temporally lean condition. I don't have any experience with a MFA, but your analysis seem possible. Have you had a fuel pressure gauge hooked up when this has happened? This isn't a very likely cause, but it may pay to rule it out. Do you have any intermittent intake air or vacuum leaks?

RACE ON!!!

 

It DOES sound like a temporally lean condition. I don't have any experience with a MAF, but your analysis seem possible. Have you had a fuel pressure gauge hooked up when this has happened? This isn't a very likely cause, but it may pay to rule it out. Do you have any intermittent intake air or vacuum leaks?

RACE ON!!!

 

bogus - No codes as of yet. It did flash me a code 36 temporarily but I have had this problem before with out that code flashing, and that code has since gone away and not come back. There are no noticeable tears or leaks in the intake tube. Base timing is set correctly for sure but who knows what the computer is doing with it after that. The coil and module are both new and from what I know of the GM HEI, its suffiecent for producing spark at the RPM range in which I operate. The wires are new as well.

Vader86 - I am pretty sure I am getting fuel and spark in each cylinder. While this problem isn't happening, it runs incredibly well. This problem only occurs under heavy acceleration. It just acts like its not getting any power and goes back to its idle state. If I am still cruising down the road at 50 when it stops being responsive, and I open up the butterflies, it pops because I have just let in a bunch of air without it providing enough fuel to compensate. I have no cat on this vehicle.

CFI-EFI - I am convinced that something is leaning out my system for those few seconds. I am thinking a possible problem with the MAF sensor or the power relay. Or perhaps the TPS but according to my scanner its range seemed okay. I will run the scanner and read both the MAF and TPS during the problem and see what the volts and grams per second read. Hopefully one of the two drop off.

I appreciate the help and ideas all. Let me know if you think of anything else. I will do likewise.

Regards,

Dylan.

 

hm... at least we are eliminating potential causes... still thinking...

I know that bastet44's 87 coupe (similar to the 86 TPI) was running poorly, replaced the MAF and things fixed themselves! It sounds strange, but, the way I figure it, the MAF has a set range of operation (I don't remember what), lets say that the top voltage is 5.0 (as I said, I don't remember), and the unit is hovering at 4.8 all the time. It won't throw a code, but it could upset the apple cart. That's what I think happened to the MAF in bastet44's car.

Just an idea.

 

Yeah thats similiar to what I was thinking. Perhaps the MAF is still staying within its set range (hence no code) but isn't supply the correct information at the right time. My friend has a MAF in his 85 (which is actually mine too) that I will swap out later today and test.

 

ah. good idea.

Gordon Killebrew, C4 God, indicated that the earlier MAFs were prone to problems. All new MAFs are upto date with current firmware and better durability.

 

I really doubt it has anything to do with the TPS. You've checked it and the voltage increases gradually and with no hitches. It sounds most like an intermittent electrical connection somewhere. Possibly a connector that breaks connection, briefly, or a bare wire momentarily shorting out? How do the wires going into the various connector plugs, like the fuel pump relay, MAF relays, etc. look? That can be a problem with older connectors.

RACE ON!!!

 

I had a simialr situation, where the connector on the coil was intemittant. I would romp on it, the car would die... ( like running out of gas) and a few pops I finally caught it one day when I was doing something and bumped the connector the engine just died. It ended up that the connector from popping it on and off numerous times installing my supercharger and my MSD caused the connector to carbon track and the onnector contacts had lost tension... I went to GM and got a new connector for itI think it was like 20 bucks, unless you want to track down the inserts and use the same housing.

Hope this helps,

Mo

 

Fingers crossed . . . I THINK I may have found the problem. As this is all in a 66 GMC, the gas tank is behind the seats, and not exactly designed for fast acceleration usage (its tall and narrow). The gas was somewhat low and I pulled the pickup to take a look at how it works. It seems that when Id hit the gas, it would push the gas up the wall taking it away from the pickup tube. I put more gas in it and it seemed to stop. A fuel cell is coming soon as anways.

I do have one question though. I noticed that my Oxygen sensor MVolts were bouncing around all over the place even while idling. Between 100 and 580. I really dont have a reference or idea of what these should be. Any thoughts?

Thanks for all your help people.

 

Here's a situation. Are you using the oversized Edelbrock runners? My friend used these on a 1988 IROC (MAF car) and encountered this issue: Take a close at the EGR runs. I distinctly remember an issue regarding these aligning with the plenum (I don't remember the exact issue, but take a look at the runner on the passenger side in the back. I think the angle was screwed up or something). Something did not line up and caused him to run really lean... His headers glowed. Damn those Edelbrockians! *grumble grumble*

 

Siggy,

At idle and evertything but WOT the O2's will go between 100 - ~980 if you take the average of the highest and lowest you should be in about the 450 mv... At WOT you owould be in the mid 800's to low 900's , 900's being on the rich side...

Mo