In February I replaced the Fuel injectors with a new MPFI spider and drove to Florida hauling a trailer with a Motorcycle and stuff at about 2,000 lbs. It ran quite well considering the sub-zero temps initially. However, I burned up a lot of fuel, so I have been trying to improve the situation. I began by replacing the spark plugs with new Iridium plugs, which were badly needed. Within days of doing that I began to experience some misfire or hesitation when idling at stop lights which got worse over about a week of short drives. Then suddenly I ended up at a dead stop on the road. After being towed home, I replaced the distributor with a new Delphi, but that did not help. After figuring out it was the Fuel Pump, I replaced that with a new Delphi and got ignition, but poorly at first. Once I got TDC figured out and the distributor in correctly, it started and seemed to run fine. However, I still am experiencing some intermittent hesitation after the engine warms up. It can get quite bad while idling at stop signs. It does not throw any codes or service lights. I have also replaced the EGR Valve, and don't seem to have any vacuum leaks. A check of the coil shows proper values according to the Haynes Manual. Fuel pressure reads steady at 56, but I did not do a leak down test. A new regulator came with the spider assembly. The vehicle seems to run fine except at idle, however I am reluctant to venture too far from home. I am thinking plug wires. If so, what kind? But, could it be something else?

So have at Blazerators: What do you think?

 

use ac delco wires for best result Also has your rotor been changed or just the cap? And what brand of plugs did you use?

 

I replaced the whole distributor unit with a Delphi. Cap, Rotor, shaft and all.The plugs are NGK, I believe. I did a water spray down test last night and saw NO indication of sparking or shorts, but it was cooler out and the engine was not very hot. In fact I was getting no indication of hesitation or misfire while testing.

 

I'd change the plugs to AC Delco as my next/first step. Also, a proper fuel pressure leakdown test is a must.

 

I agree, use AC Delco plugs #41-993. If you're not getting any DTC's in memory, there could be a problem with fuel delivery, or camshaft retard is not within specs, (must be checked/adjusted if the distributor is disturbed or removed). Fuel pressure and leakdown are easy enough to check with a pressure tester, but checking/adjusting camshaft retard requires a capable scan tool, (the El cheapos at the auto parts stores can't access the streaming data).

Fuel pressure & leakdown:
Key on, engine off, fuel pump running, pressure must be 60psi to 66psi. It must remain above 55psi for at least 10 minutes after the pump shuts off. There are no "engine running" fuel pressure specs on this engine.

Toss your Haynes in the trash, it's worthless on this vehicle. Half of the information is wrong, and the other half is missing If you want accurate and complete information, invest in a GM "dealer issue" shop manual. Ebay & Amazon usually have them new and used. They are vehicle model and year specific. ie: you will need 1999 GM S/T set of 3 manuals.

 

Update: yesterday I had a county shop mechanic run a scan with an $11,000 scan tool. Everything came out good. He cleared a MAS code that came on when we disconnected the sensor the other day and checked some things out, which lead him to believe it is some kind of fuel delivery problem at low rpm's. Perhaps bursts of ethanol as there is a fuel change over going on with a switch from winter grade gas. He suggested running a can of Seafoam through it and running it hard for a bit to let the computer resync after so many changes to the system. The was no indication of spark & ignition problems, so I don't see what switching out to new AC Delco plugs will do, over the new NGK Irridium plugs. However, the mechanic has convinced me that I will be OK for the run back to Minnesota from Florida. If i am still having the hesitation problem on idle/low speed, I may just switch plugs then and see what happens.

Thanks for the comments on the shop manuals. I agree, the Haynes manual is not worth the paper it was printed on, but it was given to me. I may try to pick up the GM manuals when I can afford them. These Blazers are quite a vexing automobile. Knowing how GM minds work would be definitely helpful.