1996 Blazer runs poorly

[mixin]

Any progress on this one? Mine is doing the EXACT same thing on my 1996 Blazer with 148k miles. FWIW, about 20k ago I replaced the intake gaskets and did a MPI FI conversion - has been running fine until now.

When it won't start, I have to try it like 10 times and then it will finally stumble and misfire long enough to the point that it feels like the computer "catches" it and the engine will rev up to like 1.5-2k rpm with NO misfire - when it finally does start, and then will idle down fine right away.

My truck also stumbles under 1/2 throttle or more when accelerating from a starting point of under 2k RPM, then it takes off fine once it gets above 2-2.5K RPM. This is especially prevalent after this hard start episode happens. Other times there is not this problem with acceleration.

I will go get some psi readings after I borrow my buddies tester to check the leak down. I will post back with the results - any feedback or other armchair quarterbacking in the interim would be appreciated!

I don't want to get ahead of myself, but could this be a problem with a worn distributor gear/bushing that is causing poor ignition timing? When I just replaced my cap and rotor, I noticed a significant amount of play in the rotor - I was able to wiggle it back and forth with about 1/4 inch lateral freeplay. Is this normal?

Thanks guys!

Mike
Grand Rapids, MI

[old_master]

Mike, lateral runout is a problem, there should be NO movement of the distributor shaft. The rotor will turn slightly, and may have slight vertical movement, that's normal, but no side to side movement. Also, these engines are very fussy when it comes to distributor cap and rotor, they're not too fond of anything other than AC Delco. Make sure you put silicone dielectric on each terminal inside and out, of the cap and rotor. Fuel pressure: Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi. It must remain above 55psi for 3 to 5 minutes after the pump shuts off. Post your results.

[mixin]

sorry, there is no side to side movement on the rotor, but I can rotate the rotor about 1/4" right to left - is that ok? Thanks,

I'll check the Fuel Pressure and get back to you. Thanks!

[old_master]

That's perfectly normal.

[mixin]

Originally Posted by old_master

That's perfectly normal.

Well that's good news at least.

After puting the engine through several key on, start, run, and wait 5 minutes cycles. I recorded a high and low average as well as some other notes. The higher readings were recorded on the third or fouth time I put the engine through the cycle.

Brace yourself.

I checked the fuel pressure and found the key on pressure to be 38-41 psi on average. While running, the fuel pressure readings would be about the same as the key on (engine off) readings. While cranking, the fuel pressure would drop about 8-10 psi and then rebound as soon as the engine started.

After shutting the engine off and waiting 5 min, the fuel pressure would slowly increase to about 46-50 psi.

Obviously, if this engine still had poppets, it would not run at all, but it has MPI now. That being said, since the swap kit came with a new regulator, is that the reason why my fuel pressure is so low, as it is just set to this lower pressure? I have never checked the fuel pressure since I put the new fuel injectors in.

Could I have a bad fuel pressure tester? I'm at a loss here of why these readings are so low.

[old_master]

Don't be doing a happy dance quite yet, you still have poppets. CMFI poppets open at a slightly higher pressure than the CSFI poppets. Most likely you have a bad fuel pump. You'll need to modify your fuel pressure tester so you can measure fuel pressure at the fuel filter outlet to be sure. Should be between 78psi and 108psi when the pump is running.

[mixin]

Trust me, this ain't my first rodeo with this truck - I don't even do a happy dance AFTER I get it fixed each time.

If I read between the lines of what you're saying, the regulator that came with my CMFI setup should have the same setting as the one from my CSFI setup. Is this true?

If true, then my pressure is WAY low. I will throw a fuel filter at it as that has probably 40-50K on it. If that doesn't work, I'll fab up the "preferred" method fuel pump max pressure tester. I hope I can get to it this weekend - if not, either way, I'll report back when I do.

Thanks!!!

[old_master]

The CMFI, 1995 & older, pressure regulators range anywhere from 55psi to 64psi and the poppets open at 43psi. The CSFI, 1996 & newer, regulator is 60psi to 66psi and the poppets open at 40psi.

[mixin]

I dont want to get off track here, but given the readings I was getting, my engine then should not start at all.?.

Since my engine does start and run, albeit poorly sometimes, should I try a different fuel pressure tester to make sure my readings are accurate?

[old_master]

Not necessarilly: your poppets will squirt as low as 40psi or so. The engine needs at least that to start. When the engine is running, voltage to the pump increases and with that comes higher fuel pressure. This is a good reason to make sure the connections, hot and ground, are good to the pump.

[mixin]

OK, more results.

I found rubber sections of the fuel lines near the transfer case and I clamped shut the return line with a hose pincher tool. Then I energized the fuel pump by running a fused jumper wire from the positive battery post to the female spade test lead near the brake booster.

30 PSI max - this during the situation when the motor wont start due after it had been run awhile. Tried with the key, same result - 30 PSI, engine stumbles and wont start.

Before I order a Delco fuel pump assy, should I check the wiring at the pump first? Or do you feel comfortable just having me get the pump?

Thanks!

Mike

[old_master]

Best to make all of the tests before jumping to conclusions.... could be an expensive jump At this point, it appears as though it's either the pump or a connection at the pump, either way, the tank has to come down to diagnose/repair it.

[mixin]

I removed the spare tire and was able to unplug the connectors to the fuel pump assy. with out having to drop the tank. The connectors appeared in good shape with no corrosion, the battery voltage was the same at the connector (with the fuel pump jumper powered as described above) as at the battery terminals. Fuel pressure was at about 38-42, pump did not seem like it was getting alot of RPM and sounded quieter than what I remember.

So, I ordered a new AC-Delco MU1735 pump assy from AC-Direct through Amazon ($236.75-shipped). I got it in four days after ordering. The pump assy came with the float arm and installation instructions for the float arm. The package also came with the truck side of the four pin plug and about 12" of wire coming out of it.

The pump did not come in an AC Delco box - no box at all. The invoice references all of the right numbers (MU1735 and 19179615).

Is there any way to tell if the pump is an AC Delco?

If I got an Airtex or other junk sold to me as AC Delco...lets just say I would be upset.

Weather permitting, I plan on starting to drop the tank tomorrow night and then finish up Saturday morning. I'll post with the results.

[old_master]

Numbers are the easy part, I would question the "no box" issue. AC Delco pumps ALWAYS come in a box, usually with bubble wrap. The only thing that might identify it is the sticker on the pressure sensor, (should say "AC Delco") but even that is not 100% proof of a genuine AC Delco pump. Some of the aftermarket manufacturers use the AC Delco sensor. If it doesn't say AC Delco, rest assured, it's not an AC Delco pump.

[mixin]

Originally Posted by old_master

Numbers are the easy part, I would question the "no box" issue. AC Delco pumps ALWAYS come in a box, usually with bubble wrap. The only thing that might identify it is the sticker on the pressure sensor, (should say "AC Delco") but even that is not 100% proof of a genuine AC Delco pump. Some of the aftermarket manufacturers use the AC Delco sensor. If it doesn't say AC Delco, rest assured, it's not an AC Delco pump.

http://www.amazon.com/Delco-MU1735-F...9567025&sr=1-1

here's the link to the pump

I'll put up some pics, let me know what you think.