88 s10 4.3 rough engine - hesitation

I had reman engine installed. Have replaced: motor mounts; EGR Valve (2 times); Coolant Temp Sensor; Injectors; O2 Sensor: MAP sensor; Idle Air Control: rotor & Cap (2 times - rotor extremely pitted after 7K miles); plugs & wires (2 times); TPS reads linear on digital and analog meters; vacuum control module; used alldatadiy to test ESC and knock sensor -- tested good; vacuum is rock steady at 18 inches; harmonic balancer is good; Timing at factory spec; advanced timing (using Craftsman light with dial -- up to 35 deg adv. and no change in engine (very strange); all cylinders firing (timing light verification). Engine starts okay, runs moderately good until warm and then is extremely rough -- as if timing is swaping cylinders - major hesitation off idle. At 2000 rpm, smooths out.:banghead:

EGR valve?

I replaced two of them -- 1 aftermarket and 1 OEM -- along with the vac. control switch for the EGR. Twp ASE shops and one dealer (along wirh me) are at a loss as to what is wrong.:shakehead

Thank you for the suggestion, though

Check your Fuel Pressure and leak down when motor is Cold & Hot. If pressure is up to spec's. Check your cylinder compression. Post back your readings.

Fuel pressure 10 - 11 psi hot/cold. Fuel leak down -- according to Alldata there is a self-release (sorry for poor terminology) of fuel pressure at shutdown. Compression is 140 psi (+/- 5%) on all cylinders. Replaced EGR Vac. Control Solenoid (tested bad by Alldata) New one tested same. EGR valve works perfectly (with hand vac pump) - looked at "pulse width" signal on dual trace oscope and it seems to vary some as duration, seems to be decent pattern. Checked timing & finding it jumpis about 4 to 6 degrees at idle. Knock sensor and ESC check good. Analog meter signal from Knock Sensor drops one volt (9 to 8) when I stike exhaust manifold - no dips in meter reading while engine is running. Rotor cap practically burned through, after 7K miles. Heavy pitting in distributor cap and on end of rotor cap lead. Suspect it is worn distributor / component looseness inside distributor.

It sounds like you mite have something wrong with your distributor. Have you check to see if your rotor and motor is at TDC. An easy way to check it is in step #5 at this link http://www.automotiveforums.com/vbulletin/showthread.php?t=881442&highlight=timing+marks You could be off by one tooth and have this problem also.

Distributor is in sync with TDC. Problem seems to have been solved by: (Entire valve train was all over map);#2 intake valve stud loose; #2 exhaust valve seated too low to seal (0.050 shim required under stud) -- adjusting took 2 1/4 turns to remove lash on exhaust and 2 turns on intake; other valves varied in seating up to 0.020. Studs are screw-in and don't enter the water jacket -- shims and other grinding on stud seats to get all valves in spec. Also found that EGR valve at 4 inches of vacuum immediately went wide open. That is the second EGR and have been told by countless shops that only OE EGR valves work properly. Replaced with Delco EGR valve. Running very stable and smooth. I thank all for the insight offered.

Now, I go to seek legal advise to go against rebuilder for not honoring warranty. :biggrin:

Glad to hear that everything is up and running. Thanks for posting back. Have a great weekend.