Cliff notes version:
SBC 350 w/ TBI with mild RV cam. Has been running rough on start up and rich from day 1. Once warmed up, would run like a top but still rich. Took it to a local mechanic. He told me I needed a new ECM. I bought new ECM. No different, actually ran worse. He told me I needed a new PROM, bought a new PROM from Howell Fuel Injection. I had the wrong PROM in my 7747 ECM. Howell fixed me up and shipped me a new PROM. Installed it and it idled like a freakin' champ, super smooth, like a brand new engine. Take it for a test drive, and it is now backfiring out of the throttle body. Take it back to the shop and tell them what's going on. They call today and tell me its fixed, they adjusted the timing, so they say. Runs great down the road now, but idles rough as hell, and is crazy jerky after you let off of the peddle. The tach is bouncing at idle 200-300 RPMs and will hesitate, and sometimes just die. I hooked up my scanner and all my values are within range EXCEPT for my MAP, it is showing around 10 IHg and the range is 29-48 kPa. Fuel pressure is about 11.5 psi.

I'm at a complete loss as to what to do now. Any ideas?

Nick

 

I looked at my scanner again noticed it gives the MAP readout in IMg (inches of Mercury) and gives the range in kPa (kilopascal). When I converted it, my 10 IMg is equal to 33.83 kPa and the MAP range is 29-48 kPa, so I am within range. Don't know what else to look at.

nick

 

was the cam approved for use with TBI fuel injection, GM TBI is very intolerant to engine changes. It does not have the ability to modify its fuel mapping very far at all. What is timing set at? And how did they set it?

 

I do not know about the cam. When I bought this vehicle, it already had the motor in it. I installed the TBI on it as it did not have a carb or fuel injection, obviously I opted on the TBI route. I was told they set the timing back to zero, I was not there when they set the timing so I was not able to watch them to see how they set the timing.

My question is, I had it running before I took it to them fairly smooth, now its all I can do to get it to idle. Once I give it throttle, it runs great, but the idle sucks.

nick

 

Since the idle surges as you say 2-300 RPM the first thing I would do is look for a vacuum leak. You seem to mention MAP and low vauum then OK vacuum. It may be a vauum leak especaillly to MAP. MAP should be hooked up to center vacuum port on rear of TBI. HTH!

 

I've checked my vacuum hoses and the MAP is hooked up to the rear port of the throttle body. Would improperly set timing cause this surge and rough idle?

 

Not normally but it will with a big cam!

You have no information, like a cam card, for the cam?

 

Nothing for the cam. All I was told was that it was a mild RV cam. Thats all the info I have on it.

nick

 

Mild RV Cams are not an issue with EFI. They still would need a chip tune to get all the performance out of them. But should run fine with stock chip.

I'd still look for vacuum leaks for surging idle. Check timing again with wire disconnected and set to whatever the engine calls for, maybe a few degrees advance to try and get a smooth idle. But don't do any WOT runs with timing more than stock ECM/chip calls for as it will rasise total timing above what's safe.

 

just reread your first post and you have a howell chip. They have been around awhile so you probably have a good chip tune there for an RV Cam. They should have told you what to set timing at with their chip. If not ask them! I'm sure it's not stock timing and they made adjustments neccessery in chip to compensate for idle timing.

 

I spoke w/ Howell fuel injection this morning and they ran through all my readings with me on my scanner. The only thing they said that is out of spec is the Block Learn, which at idle during closed loop is 108. As soon as I give it gas, it jumps up to 124-128ish, but if I let of the gas, it drops to 108. All my other sensors are where they should be according to my scanner range and Howell. They told me to set my timing at 10* before TDC, which I did.

Howell had me restrict my fuel to about 9psi to see if that improved my block learn, but it did not.

I thought SBC TBI motors wanted around 12psi of fuel delivery? Maybe a new fuel pump? Its only a year old. I just hate throwing money at something when I don't know if that is the problem or not.

Nick

 

They are around 12 and better at 13. What he did by having you reduce pressure is prove there's not to much fuel. It would also lead me back to a vacuum leak. Vacuum leak causes the MAP reading to become a lower vacuum and it thinks there's a load, so it increases fuel. This becomes more apparent at idle when your usually in open loop.

 

I'll double check all my hoses and see if I can find a vacuum leak somewhere. Any quick and dirty ways to check for a leak?

I've also taken my timing to about 12* BTDC and my idle has smoothed out somewhat and my block learn numbers have gone from 108 at idle in closed loop to around 113.

Thanks for your help!

nick

 

the way I check for vacuum leaks is very dangerous but usually finds them after carb and choke cleaner shows nothing. So remember the dangerous part before you try, do it outside, no wind, long arm and short bursts on specific parts. OK remember the dangerous part? starting fluid! Please be very careful.