Questionhey thomas,
I have a 1988 chev silverado with the 5.0, 305 tbi, auto, air,2wd. When its cold and i start driving and i step on the gas to speed up from a dead stop or accelerating around a corner, it almost dies, it seems to take off fine and when it shifts into second it falls flat on its face like it is flooding and you keep stepping on it and sometimes backfires (in winter), sounds like its backfiring through exhaust, but takes off again, idle lopes also when it is really cold until it warms up, In the last 5 months i have replaced (with delco parts) the egr valve, iac, distributor complete with cap/rotor/ ign module, electronic coolant temp sensor, swapped tbi unit, /, i replaced the fuel filter, Put dual evhaust on with no cats, i flushed the injectors, i installed new intake manifold gaskets, i replaced the o2 sensor, it has a new air filter, i did a fuel pressure test (13psi, spec says 9-13), put a vacuum gauge on it(18 inches or so), put a oscilloscope on it and all cylinders are fairly even, egr solenoid holds vacuum, plugs are gapped at .35, compression 160 at the front and 148 at back, timing steady at 0, transmission rebuilt last year, i checked the timing chain for play, it is a double roller and is in spec. thanks for your time and patience, if this is a common problem i dont know i work at a ford garage, have had a few guys look at it but they are still unsure as well, if you could help me out with some advice that would be great, i am out of ideas, thanks again. mike
AnswerMike,
sounds like you've done a ton of work here! I would try a couble of things:
First, since it sounds like a fuel problem I would try to monitor the fuel pressure while to problems is happening, durring a road test. I know they are a pain to hook up to but one of your local parts stores should have a gm (G109) style fuel filter that had a shrader valve attached to it, it sells for about 30.00 Can. so its a great tool to have even if you only use it a few times. Anyways just make sure you don't loose pressure when it acts-up.
I would also test the MAP sensor on this truck, make sure the vaccum sourse it manifold vaccum and the hose is not plugged. Also use a lab scope or DVOM to monitor the signal to the PCM. You should have 5V reference, a ground, and I believe the middle wire is the signal.(double check) Hook uo the DVOM and use a vaccum pump to slowly build vaccum, monitor the signal should rise smoothly to 5v no drop-outs or dead spots. This is just a simple potentiometer style sensor not the frequency type found in most FORDS, so its pretty easy to test.
I would also make sure the TPS is within spec. Test basicly the same as the MAP just open throotle slowly(engine off) and make sure it sweeps to 5V without drop-outs or dead spots.
MAP sensor problems are pretty common and I have seen a few TPS cause some problems. Another sensor that can cause some strange drivability problems is the Knock sensor its tough to test without a generic scan tool......if you have on just make sure its in working condition.(more info on this if you need)
Another thing.....you said the timing was steady at 0 but it should only be "Set" at zero. when its running and you rev it up it should show definate advance! Make sure the "Timing Set" connector is plugged in. In these trucks its usually behind the plasic cover on the fire wall near the fuel pump relay, one wire connetor usually tan with white stripe, has to be unplugged to set timing. With the connector unplugged inspect the timing should be 0 then turn of the truck plug it in and reinspect, timing should advance with engine speed. I have also seen some elcectronic spark control modules fail, if your truck has one, small flat module mounted on fire wall near distributor.
Hope this gives you some more ideas.......Let me know how you make out, or have any more questions!
