kael

Hey all, time to get opinions.

1987 z52 w/L98, 56k

Recently, drove on the interstate for about 40 minutes. Temps were fine, about 198 for coolant. Weather was a little hot but not bad.

Ran fine (and made a few "passes" threading through semi quad train ). Can't recall how I drove exiting interstate, didn't notice anything. Another 2 miles at 50 mph, stopped at a store for about 10 minutes.

Started back up, stopped at a traffic light for a left turn. Green, go, bleah. Power was down, mid to 2/3 throttle. Lower throttle was fine. Turned to an isolated road and tested some more, same.

I've been chasing this since, in garage except for test runs . Current state, if I start with clean ECM (battery disconnected/reconnect), throttle is great, all works as expected. Mid-2/3, WOT fine, until engine heats up, about 180 oil/180 coolant. Then problem is back.
Mid-2/3 remains slow, chunky, up/down. Sometimes WOT is okay, sometimes not. Seems to depend on the throttle pattern I use. One weird thing I noticed, from 2 runs, is mid has a patterned drop/up, like a wave, once per run. Definitely makes me think fuel pump, but...

Through all of this, light throttle is fine, 60+ mph.


Bits:
Fuel pressure: 20psi at ignition on, 35psi running. This is at idle. (need a longer hose for driving, any pointers as to what?)

Fuel filter: replaced, didn't know status of old one anyway.

Spark plugs, distributor, wires: all replaced 10 months ago

Temp: oil and coolant below 200 unless in town stop/go.

Compression: Haven't checked, would kill me anyway if down.

Exhaust: no smoke, no strange smell

Codes: after initial problem, had a 32. I've had this before, wiggling the wire on ERG temp switch makes it go away. I do have a new switch on order (for $50! Grr.) No code 32 since the first time. Additional code, 36, I've had for 10 months, keep meaning to chase. Again, with clean ECM, it doesn't know about the burn issue until second start, so MAF is used. I did runs with code 36 in place as well, same thing.

What else to chase? I was going to test fuel volume, but don't have the proper hose yet. I'm on a budget too (like many these days).

Really appreciate having good people to query here, try to answer the questions I can too. Thanks!

antfarmer2

Slow o2 sensor?

kael

Okay, time to check the pressure at runtime, spend a couple dollars on a longer hose for my pressure gauge.

kael

That should pop a code or affect idle, don't you think?

Idle, start and low throttle works fine, open or closed loop.

antfarmer2

Didn't on my 94. my idle was fine it would just bog sometimes and even a fart every now and then.

Kevova

The DTC 36 relates to MAF burn off relay. I would replace both the MAF power and burn off relays. TSB 87-657-107. also mentions PCV Vent hose blocked or leaking so I would look at that also.

JrRifleCoach

FP @ idle is OK 35-38 Should jump up when you blip the throttle 40-45.
Double check the vacuum lines to the FPR Check for leaks and check for vacuum
Also check for presence of fuel in the vacuum line. Should be none.

O2 sensor is cheap enough to replace and forget about it.
However be careful with the new EGR temp sensor. Easy to break.

kael

Hmm, another thought. I just replaced the ECM three weeks ago, possible that's started going south?

How is an ECM validated by the home mechanic?

I'll recheck fuel pressure, pretty sure but not positive I blipped the throttle and it stayed right at 35.

Slow o2 might be it, I was doing a run with Seafoam (yeah, might as well try) and noted if I went to mid, it muttered and then I held the throttle, in a second or so it took off like it should. Hmm.

I want to test the o2 but don't have any decent back probes....yet! Odd, few locally sell those without asking for gold. EDIT: Wait, NAPA, in town has an o2 sensor for $20? SOLD!

Kevova

Non OEM/ Delco ecms can be iffy, Why was ecm changed? There really is no way to test GM ecm other than by symtom, DTC or substitution. FSM will have pin out info related to connectors, but not input and result.

kael

Old ECM failed, no start, no code 12. I replaced with a remanufactured Delco, it's about a month since I ordered. I guess I might warranty if o2 replacement fails.

kael

Heh, for my First time inspecting a fuel pump, ah no, not 30 minutes.

I'll put on my winter list. And spend my core credit at Rockauto.

kael

Okay, finally got a driving fuel pressure test, same, 20 psi before start, average 35 psi from there in a variety of throttle position. FSM lists '87 fuel pump pressure correct at 34-37 psi.

Time to change the O2 sensor.

kael

O2 changed, no fix. *sigh*
Think it's time to read the spark plugs, low throttle sounding a little off now.

antfarmer2

Have you ran the fuel pressure test yet when driving? You can get a loaner and tape it to your windshield. Be careful closing the hood so you do not crack your windshield.

Did you pull the battery cable to reset it?

kael

Yes and yes, many times.

antfarmer2

Ok I would look into the fuel pressure and take the coil and ignition module in next for testing make sure to use thermal grease on the mod. you can buy a longer hose that will screw on to your pressure gauge.

Kevova

The Key On 20 psi then 35 psi when it's running I'm just not getting. I would check connection between fuel sender and pump for leak. If pulsator is present replace it with a piece of fuel hose.

ex-x-fire

Key on/engine off fuel pressure should be higher then when running because there's no vacuum to the pressure regulator. When you crank open the throttle, you loose vacuum to the regulator which increases the pressure.

kael

Thanks folks, my fault, missed reading fuel system diagnosis in FSM, which clearly indicates LOW pressure at ignition on/engine off is bad.


Reading is Good.

Still going to pull the plugs, in case they tell me "Hey, there's this other problem too.".

For '87 FSM owners, see 6E3-A-18 "FUEL SYSTEM DIAGNOSIS'.

kael

YES!! YES YES YES YES!!!

Finally got my fuel pump motor, replaced it (first time!) and 40 PSI at ignition on/engine off!!!

Started right up on the first try! Few test drives, zippy zippy ZOOOOM!