Chev TBI issue

[Digger 5]

I have a complete 92 Blazer TBI 350 conversion in my 88 YJ. I've had a problem lately with a 'miss' at startup. It doesn't matter if the rig is cold or warm, when it's started it runs rough for a few minutes. You can hear the miss for a few minutes then it clears up. To me, I feel it's a sensor or computer issue but because I hadn't done any tune up for a while, I replaced the plugs, wires, dist cap and rotor with no change (but I only half expected one). I have also changed the water temp sensor.

Any ideas on where to look next?

 

[crash2]

I am thinking lean miss.

What fuel pressure are you running and was it a kit?

 

[jeeptech 1]

Make sure the TBI base gasket is not broken causing a small vaccum leak and one other thing that happens sometime is the distrubutor shaft housing (bushing) get worn out This some times will give you a miss fire condition cold but most of the time it is a stall or intermittant on start. Yes check fuel pressure first, Tbi gasket, O2 sensor, Distrubor shaft and housing.

 

[Digger 5]

I can't remember the fuel pressure but it is the stock 4cyl electric fuel pump. I remember when I started the swap the pump I used was a common pump for the system. It works fine warm at WOT so I don't think the fuel pump is an issue.

Vacuum is steady and I'm not showing a leak.

 

[crash2]

A leak will typically cause a higher than normal idle.

Try this. Get a can of carb cleen and remove the air cleaner. Start it cold and just mist some carb cleen into the throttle body to see how it reacts. You should be able to rule out a lean miss this way or confirm it.

 

[jeeptech 1]

did you spray wd 40 around the base of TBI and see if the RPM raises if it raises TBI gasket leaks.I'm still thinking Distrubor shaft housing and shaft this is a common problem on TbI engines with some miles on them.

 

[crash2]

did you spray wd 40 around the base of TBI and see if the RPM raises if it raises TBI gasket leaks.I'm still thinking Distrubor shaft housing and shaft this is a common problem on TbI engines with some miles on them.

Not real common on the v6's and 305/350's from what I have seen. But was common on the 454's--they always chewed up the bushing. But ya it would not hurt to check it...

 

[Digger 5]

I'll do the carb cleen and wd40 tests. When I was in the dist I checked a visible bushing (a pick up for the module?) and it looked great.

 

[crash2]

I'll do the carb cleen and wd40 tests. When I was in the dist I checked a visible bushing (a pick up for the module?) and it looked great.

Take the dist shaft and try to move it side to side. A "little" play is normal. What I have seen when they wear brad the magnetic assembly will start to make contact with the outer ring with fingers.

 

[jeeptech 1]

Also did you check the vaccum line to the MAP sensor? Just a though. Are you getting a miss fire code or and codes. Do you have a Scanner? what is the block learn readings?

 

[TreeClimber]

What about swapping out the ecm with the spare one I gave you last year?

 

[Digger 5]

I'll check the dist shaft and the map sensor vacuum line. I can try the computer but it's more of a bitch to check than the other things. I have not put it on a scanner. I'll do that but my check engine light hasn't come on.

 

[mr.climax]

Make sure you have you 12V constant power hooked up to the ecm. Sometimes the 12V constant gets tied into the key switch on custom swaps. Without the 12V constant to the ecm it has to relearn how to run after each start. On all the GM ecm's i have worked with all the orange wires to ecm are intended to have 12Volts at all times. One of them will keep the memory. Seriously, I'll bet that's your problem.

 

[Digger 5]

Very good to know. I'll be checking that!!

 

[Digger 5]

Since I had 2 things to check, I looked into the ECM and wiring first. Both are fine. I checked for constant 12V and it was there. I changed ECMs and it didn't make a difference.

I still need to check for vacuum leaks and the dist shaft.

 

[superglock]

Did you check the timing with the wire unhooked? Maybe check and see if the pump is getting juice when the key is in the start position. I lost a pump after Jamboree last year and it would make it start hard but it still ran good other than it would hesatate off idle.

 

[Digger 5]

Did you check the timing with the wire unhooked? Maybe check and see if the pump is getting juice when the key is in the start position. I lost a pump after Jamboree last year and it would make it start hard but it still ran good other than it would hesatate off idle.

Mike-
It doesn't hesitate off idle. All it does is run a bit rough when you first start it, hot or cold. It will drive a bit rough for a couple of minutes then it runs fine.

I just checked for vacuum leaks around the TBI base and around all vacuum lines and found nothing. I shot carb cleaner down the TBI and it just bogged down until it cleared out.

Next stop, distributor bushing......

Although I haven't found anything yet, I really appreciate the help.:beer:

 

[Binder]

Next stop, distributor bushing......

Doesn't seem that a dist bushing would be the problem. It would act up all of the time not just when first started....
I would check the codes next. Many codes the check engine light will not come on.

 

[TreeClimber]

My YJ does the exact same thing. It's due for a tune up, as I haven't changed the plugs or wires since I built the engine in 04 or so. Misses just a little, on cold start up, and always goes away. It's my opinion that both our problems are more likely due to ignition than fuel. With throttle body injection, the fuel is delivered much like a carb. That makes me think that a 'rich' condition or a 'lean' condition would not give that type of intermittant miss. And all the electronics of the fuel injection system are simply designed to manage the air / fuel ratios. So a failure of the fuel injection sensors would simply make it run crappy with great regularity, not intermittantly. I could hypothesis that the condition is too rich at startup, but you should be able to smell the unburnt fuel in the exhaust. I suppose it could also be too lean at startup... dunno.

So I'm back to wondering if it's tune up related. Perhaps one of the (new) plugs or wires is got too much resistance, is improperly gapped, not quite 'clicked' on the nipple, etc.... or maybe the coil is getting a little weak, and isn't firing the plug when the engine first fires off (it's rich during startup). Maybe the 12v wire to the coil has some resisitance...

 

[Digger 5]

Tony-
Like you, I feel it's ignition related. Changing the wires and plugs made no difference.

Binder-
I haven't been getting a 'check engine' light but I'll be checking for codes anyway.

Thanks for the help.