1994 ¾ ton Suburban, 454, TBI, OBD1
Initial symptoms presented as what seemed to be a miss at light cruising throttle and a rough idle.
Based on this I investigated the ignition components: there was one spark plug which seemed like it was darker than the its neighbors, so I condemned that cylinder as the one not firing. The cap and rotor weren’t new, but weren’t terrible either. I gave it a new cap and rotor. No change. Then I replaced the plugs and wired and checked the gap on all the plugs. No change. While the plugs were out, I checked the compression, since this is a very high mileage engine.
Compression ranged from a low of 95 to 125. Most were right around 120psi, so no dead cylinders. Still seeing no improvement, I swapped ignition modules. No change.
So at this point I was stumped and decided it perhaps wasn’t an ignition problem after all. I thought it was maybe fuel related that perhaps.
I should add at this point that never were any codes presented and the only previous work done was a new IAC earlier this summer – after which it ran perfectly.
So in any case, I made a visual inspection of the injector’s spray pattern. It looked good – even and no dripping after shut-off. I checked the fuel pressure, since there’s no schraeder valve on this TBI setup, its kind of a PITA but it was a steady 30 psi in the driveway. I do some towing with this rig without issue.
I began to think perhaps it was not coming out of open loop due. I looked at the temp sender and it did not seem to be giving the proper resistance for the temperature I was measuring with my IR thermometer. i.e. according to charts on the internet, the resistance is showing I should have a temp of around 68*F.
I got a new temp sender. It behaved somewhat differently from a cold start. Monitoring the voltage, it started out at ambient according the chart and started falling towards going under the magic 1v number. This corresponds to around 158*F where it should tell the ECU to go into closed loop. Instead, as it approached, it unexpectedly jumped up to 2.33v, corresponding to around 95*F – where it stayed. The problem persists.
Next, I learned that you could monitor the open/closed loop status via the CEL on the dash. But before I did that, I tried clearing the ECU and initiated an idle re-learn incase the problem was IAC related. No change.
So I carried on and with the monitoring of the CEL from a cold start. What I found it that it was coming out of open loop, and much more quickly that I imagined it would based on the engine temperature shown on the gauge. And yet still, no change.
Now through the whole brief time I’ve owned this rig, I always thought the exhaust smells a bit rich, but who know. It certainly doesn’t smell as clean an fresh as some of my other cars, but it’s not bellowing rich black smoke either.
I’m stumped, so today I pulled the throttle body off the intake. The pic shows some type of foam in one of the passageways. It looks like another part might have recently broken off near that same throttle blade. I wasn’t expecting that to be in there at all. What gives? I’m still stumped.