1995 Blazer, bastard OBD1.5.

Start it up and itll bounce from 400-800rpms, then hold at 2k or so for a few secounds then back down to idling rough.

WIll do this 4-5 times before "warming up" and running fine. It has a CEL on it I cant get a scanner to read it. Ive tried a new IAC and a new EGR valve.

Any ideas on where to go from here?

 

1995 with OBD1.5 has EDI ignition, (the plug wires attach horizontally to the distributor cap). Install an AC Delco cap & rotor. Put a dab of silicone dielectric grease on each terminal of the cap. While the cap is off, make sure the vent holes in the bottom of the distributor housing are clear. If it has filters in the vent holes, remove them.

Without codes and streaming data available, it makes things extremely difficult to diagnose. Try checking fuel pressure and leakdown. Key ON, engine OFF, fuel pump running: pressure must be 55psi to 61psi and must remain above 50psi for at least 10 minutes after the pump shuts off.

 

A cap and rotor wouldnt explain the random reving to 2k. I replaced both less than 2k miles ago.

One of the other techs is thinking coolant temp sensor but Im looking for a better answer than to just keep throwing parts at it.

Ill test the fuel pressure in a few and report back.

 

Correct, the cap & rotor will not affect the surging idle, but could be the cause of the rough idle. Could also be a problem with camshaft retard out of spec causing crossfire in the cap.

ECT sensor could be a possibility. If it's not calibrated correctly, it might not set a DTC but still adversely affect fuel mixture. Really need a scan tool to tell if it's accurate, and make sure it transitions smoothly as it warms up.

 

Fuel pressure was 56psi, after 20 min or so its down to 44.

Thats a step in the right direction, but doesnt explain if its in the spider or the little check valve in the pump, going to start with the spider as its easier to diag

 

Check maximum output pressure and leakdown at the pump, (or fuel filter) first. The results will tell you where the problem is without taking anything apart. You need to make sure you're getting good fuel delivery to the injectors.

A 20 minute drop to 44psi may be a bit excessive. The regulated pressure at 56psi is close to the bottom of the allowable range, which means volume is also near the low end.

If you get a little creative with plumbing, you can make a tool that will isolate the pump from the injectors. This is one that I made a few years ago for 1996 and newer. On your 95 you'll need to use different fittings at the fuel filter.

 

I made a guage thats not quite as fancy as yours, but works.

Priming the fuel pump 3-4 times gave me roughly 70psi with a new fuel filter.

After about an hour it is now down to 42ish psi. Going to let it sit overnight and see where we are at.

 

Perfect! Great job! I like it! All for under $20 ! We ought to have Kyle sticky it!

70psi is low. It's enough to satisfy the fuel pressure regulator, but fuel volume will be low. Could be a weak pump or poor connection. Do a voltage drop test at the pump and make sure the ground is good. If either circuit is poor, it will cause the pressure to be low.

The only thing you're concerned with on leakdown, is that it remains above 50psi after 10 minutes.

 

Its at roughly 30psi after sitting overnight. 10 minutes is the only number we are concerned with here?

Just reprimed the pump, it was 75psi and after 10min was at 65psi.

 

Is there supposed to be o-rings on the end of each injector?

I installed the new filter and with the truck running I was getting a tad over 50psi of pressure, with it off, after 10 minutes I was down to 33psi. I cannot find a visable leak anywhere. I popped the plenuem off, cleaned and dried everything, put the spider pack in the parts washer and dried it off, so everything is DRY, prime the pump up and cant find a single leak anywhere.




Couldnt see any visual leaks, going to clean everything really well and try again.