Ok, I'm working on my first attempt at TBI (or any EFI for that matter). My background as a shade tree mechanic involves mostly classic GTOs, Corvettes, etc. with carbs. Anyway, I've got a complete TBI 350 out of an 89 Suburban and transplanted it into a 85 Diesel Chevy 1 ton to build an off-road toy. This is using the 7747 ECM and complete factory harness. It ran great when I first put it in and fired it up. Ran great for several weeks. However, it recently started surging between 700 and 1250 rpms at idle. It has similar but more pronounced behavior at higher rpms (when I open the throttle). I was at a loss to figure out what was up but luckily someone pointed me to WinALDL. Sure beats buying an expensive scanner that only shows 1-4 data points. Problem is, I don't know what to make of some of the data values I see. Things I recognize (that make sense to me, like temp, TPS, etc.) seem fine. However, IAC is running 60-145 (mostly 145) and, reading here from searches, that seems VERY high. Maybe I've found my culprit? How can it be checked? Is the valve bad, or is there maybe something else causing the ECM to cause it to behave that way? Sorry to ask what is probably a very basic question but, I have searched far and wide for info that will help me understand the WinALDL data, with no luck. A complete data log for a properly functioning system (open and closed) would help out a great deal in identifying values that indicate a problem. I'm learning but it is a slow process... Any help will be appreciated.

Hmmm, I just found where the MAP should be around 1.5 Volts but my logs are showing it bouncing around from 20 up to 65! How is that even possible? I guess I have a fried ECM or a bad MAP?

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Doh! I just went and checked this again. UI shows the right voltage. Looks like the log is raw data, not converted to volts. Map looks ok at 1.5-2.0 volts at idle (which is surging up and down).

Send me your data log file for at idle and I'll take a look at it. Remember to have all the boxes checked and use US measurements, not metric. Either e-mail it to me or post a link to it in a reply. I'm pretty good at figuring out what's going on if you can data log ALL the info. The flags are usually a good idication as to why the IAC is behaving the way it does. Some things like A/C on or park/neutral switch, well make the IAC open up a little more causing a slight idle lean condition and surging if not enough fuel is avail.

Well, I worked it out on my own. I was taking the TB off so that I could remove and check the IAC visually. When I cracked open the high pressure fuel line, there was no pressure even though I had cut the engine off only minutes earlier. I don't have a bed on right now so I pulled out the pickup/pump and found that the rubber hose was deteriorating (it looked fine a few weeks ago when I put it in) badly. Swollen, spongy, splitting on the ends under the clamps, etc. When I pinched it in the middle, a fair sized hole became visible.

So, down to the parts house and I have a new Delco pump, fuel filter, and sock. Installed them and it runs like a champ. One other thing, the fuel filter had broken loose inside the canister. Never seen that before. I'm sorta glad it was running bad, otherwise I would have likely ruined the injectors because the fuel was not filtered at all. It's running fine now but I'm still concerned about what may have gotten into my TB. This filter and pump were from the donor and I was planning on replacing them after the first tank or so anyway (after they had time to ingest any junk, hmm, maybe that's what got 'em?). Now I wish I had gone on and changed them to start with.

No more problems that I can tell. I've been planning on installing a permanent fuel pressure gauge, now I have even more reason to do so. I wasted enough time fooling with this to have paid for 2 or 3 gauges. Oh well, at least I've learned a little about the ECM/TBI and WinALDL.

I'm still including my log files for the first run after changing the pump. I started logging just before cranking and let it idle until it warmed up and the idle went down. Then I brought it up to about 2k and held for a short time, then up to around 3k for a few seconds, then back to idle. After that, a few short blips on the throttle to about 3k before shutting it down. I figured that would give a few different things to look at. I have no idea whether it is actually right or not, all I know is that it seems to run right setting in my back yard. No tags on my truck so it I can't run around the neighborhood too much. If you could look it over and let me know what you think (if anything looks out of whack) I would really appreciate it. Just to let you know, right now I don't have a VSS (not moving anyway) or neutral safety switch. Don't know if that will matter considering it is setting in my backyard but, thought I would mention it just in case.

Thanks much for any help you can offer.

Hmmm, looks like the attachment didn't work.