***Not an Fbody warning***

Alright, Volvo wagon with TBI 350, th700r4. Stock '89 truck engine except for Megasquirt II v03 fuel injection computer and LC-01 with Bosch wideband 02 sensor. I've had a recurring problem even with the original ECM, before the MS swap. Letting off after part throttle acceleration (say, going through a yellow light) will often lead to a stumbling "running on six cylinders" sort of moment. It lasts anywhere from about five the 30 seconds, where the car won't idle and runs terribly rough. Putting it in neutral and revving it a few times seems to "cure" it. The WB O2 shows it's running rich not lean during these moments, so I'm assuming it's not fuel starvation.

Things I've replaced since this prob existed.

plugs
wires
cap
rotor
O2 sensor (bosch 5 wire wideband)
MAP sensor (now onboard Megasquirt)
knock sensor is currently bypassed for simplicity sake


I checked the coil and it's still within the spec listed in the Haynes manual.

I'm basically down to thinking something is up with either the IAC or the HEI module... Any ideas anyone?

thanks!

M.

 

How rich is it? Mega rich conditions will cause a stumble/chugging due to the fuel reaching the rich burn limit. Youd have to be at less then 9-9.5:1 for this to happen, though.

 

Hard to tell 'cause it's stumbling so bad, but I'd say 11-13:1. Definitely not lean, and it sounds different than when it's running too lean too (I almost ran out of gas one day and got to watch the O2 go lean then).

M.

 

Hey Volvo

1) I would check TPS sensor for a dead spot over full range of throttle shaft rotation. TPS is a variable resistor and over time they wear-out. Worn spot may cause TPS voltage to jump making the ECM (MS or GM) think that throttle shaft is at WOT or else where.

2) Check injectors for spray pattern under throttle-up throttle-down (in park naturally) with a timing light. Any dripping, etc is an alarm.

3) Check fuel pressure stability - it should be rock steady unless you have VFPR

4) Vacuum leaks, cracked TBI pod, leaky TBI gaskets all may cause performance issues.

I hope this helps. A V8 powered Volvo hmmm - that you should be interesting.

//RF

 

Well, at long, LONG last I figured it out... it was the damned EGR valve. It worked well enough at idle but was sticky so it'd get stuck up under acceleration. I tested it on the vehicle (I'd push up on the diaphram and the engine would stall) but it didn't show how sticky it was... so just something to consider for others dealing with similar woes.

Next time I'll take it off the vehicle to test... when it was off and next to the new one it was obvious how clunky and sticky it was.

M.