Bad Fuel Pump ???

[Gunny]

Quick disclaimer ... I've already searched our forum & have read as much as possible on fuel pump issues, replacement, etc.

My situation:
90 ZR-1 w/128,000 miles. I have all the service records since 70K miles and the fuel pumps have not been serviced/replaced during that time so I'm assuming that I have the original pumps.

Symptoms:
started about 2 weeks ago after not being driven for a couple of months. Engine started normally, no indication of problems, but the engine began "bucking" and cutting out/missing at 3000 rpm as if it had no fuel. It did not seem to be an ignition problem. Service engine light came on briefly but went out almost immediately. The engine missing occurs above 3000 rpm whether under load or not. After driving for 10 minutes or so, the missing stopped and everything seems normal ... full power, etc. A/C plugs & Accel injectors are all quite new, with only trips to BG & Carlisle ... fuel filter also fairly recent. Except for trips outside of my immediate area the Z always gets premium with NO ethanol (I have local access to non ethanol premium).

Analysis:
checked for loose vac hoses but no problem there. Put fuel pressure gauge on the rail and I'm reading 38lbs when key is first turned on. With engine on I have about 38-39 lbs at idle and with slightly higher RPM I get about 41-42 lbs. With engine not running and key on, the fuel pressure starts at 38 lbs but with each cycle of the pump turning on the pressure drops until it's down in the low 20's. If I charge the fuel rail up with key on and get the 38 lbs and then turn key off, the system holds the 38 lbs for quite a while (i.e. takes some time for the pressure to leak down). This is why I'm suspecting a fuel pump issue ... shouldn't I have 50+ lbs of fuel pressure?

In my mind it appears that I need to replace the fuel pumps but have I missed anything? Here's where I need the gurus to chime in ... Pete? others? any/all help & advice will be appreciated.

If I do need fuel pumps what's the best approach?
Rock Auto Part #P240KC. (Performance Electric) - less than $30 each
complete OEM kit - $345 from Jerry's Gaskets.
Obviously, a pair of pumps for $60 is more attractive than any more-expensive options but I don't want to make a bad decision on this issue.

I read the thread on the Bosch replacement requiring minor modifications (good writeup, btw) but I think I would prefer to replace without cutting, modifying, etc. I've never changed the fuel pumps on a Z (never have seen it done either) but it looks fairly straight forward. Any advice and/or tips would certainly be helpful.

Thanks in advance ... hope to see a bunch of good replies so I can get the Z back on the road. Any harm in driving it before I make repairs/replacements? ... or should I leave it parked? I just drove it again this morning hoping the problem had disappeared on its own, but alas, the first few times I hit 3K on the tach the engine began to cut out ... but after about 10 minutes all the symptoms were gone and everything seemed normal.

[zr1mom]

Hi George, my first thought might be the Fuel Pressure Regulator? Since the check engine light came on a code may have been set that might give you a hint as well.

[Gunny]

thanks ... hadn't thought about the FP regulator. Haven't pulled the codes yet but will do that also.

[ZZZZZR1]

George

Yun and I changed fuel pumps recently (for preventative maintenance) and I think we used suburban pumps @ $75 each. Check with Yun and see where he bought them.

Jim Voter might have the best way to test them



David

[Gunny]

Code 56 (assuming I pulled the code correctly) Any ideas?

[DaveK]

Code 56 - port throttle vacuum sensor?

[zr1mom]

Code 56 will be set when:
1. Vacuum sensor signal voltage is less than .46 volt at any time
2. Vacuum sensor signal voltage is greater than 5 volts at any time

This checks to see if there is a constant vacuum signal on the sensor at idle.
If the ECM recognizes the low vacuum signal, the ECM and wiring are OK.
If the ECM recognizes the high vacuum signal, the ECM and wiring are OK.

[zr1mom]

You might also check your secondary butterflies and make sure they are opening and not binding shut or sticking.

[Pete]

It sounds like secondaries if it's bucking/cutting out at 3k.

A quick simple test.

The MAP sensor under your ECM get a long piece of vecuum hose connect one end of the hose to the MAP sensor (under the ECM) connect the other end to your vacuum pump go for test drive.
If the bucking/cutting out goes away then look for vaccum leak or a bad electric secondary solonoid.

This is just a quick unorthodox diagnoses not a fix before you start throwing parts at her.

Pete

[Marc Haibeck]

Hi George,

Code 56 means that the secondary vacuum diagnostic sensor has an electrical fault. When code 56 sets a 3000 rpm rev limiter is imposed.

The sensor is located under the ECM mounting bracket. Maybe the electrical connector is loose. If there is a good electrical connection the sensor has probably failed. The sensor part is available because it is used by many other vehicles. GM PN 16231141.