1995 Z28 with about 120K miles on it.

When I first start the car cold I just turn the key and let it fire up and leave it until it's warm, occasional blipping the throttle.

I'll drive it somewhere, length of trip doesn't seem to matter, and shut it off. I get back in, fire it it up and it has a miss, like having fouled plugs for example. I give it a bit of throttle to see if it clears up while stationary, then drive off and as I go it starts to clear up and finally the car runs just fine. This behavior has been getting worse over the last 1000 miles or so.

I saw a couple threads discussing this sort of behavior, one that focused on the fuel pressure regulator as a possible culprit and I plan to replace it but wanted to get ideas for other possible explanations.

Another data point to throw in. For some time now, probably a bit longer than the miss problem outlined above, when first starting the car cold I get a ratcheting sound when increasing the throttle, more throttle and it gets faster, like a loud ticking sound, but the engine is not missing, just that sound. This also goes away after a short time, but, even when the car sits off for maybe 15-45 mins I have had that sound again when starting up but it goes away much quicker. This does not happen every time, it's intermittent, but is definitely becoming more frequent, both of these problems are in fact. For all I know they may be related.

While I was at a shop getting something else done I fired it up and someone mentioned that it sounded like(the ratcheting sound in issue 2 above) the "cold start valve" was going bad? Do these motors have such a thing? I have the GM shop manuals for the vehicle and plan to research them but they are not currently in my possession, I won't have them back until the first of the year. I wanted to try and get a sense of what may be happening before then, hence the reason for the post.

Thanks for any and all help and suggestions.

 

No cold start valve on an LT1. The PCM richens the mixture for cold start based on coolant temp. What does happen on startup is the AIR pump runs for a couple minutes. Is that possibly the sound - either the AIR pump itself or the check valves on the exhaust manifolds?

Have you scanned it for codes?

 

Possibly, I saw the other thread about the air pump possibly being the cause of some noise but I haven't checked it yet. It does sound a bit like a belt squealing, or actually a pulley itself I should say. The noise sounds as if it's coming from the rear driver's side of the motor, but that location is not exact.

I'm going to try the pulling of the air pump relay method of diagnosis tomorrow and see what happens.

And no, I have not scanned it for codes, that is also something else on my list for tomorrow. It's a second vehicle and I have admittedly not been paying attention to it of late.

Just to add, the sound is like a clacking more than a squeal, a mechanical sort of noise, like a loud ratchet with very few teeth.

 

I had a few minutes today to stop by my friend's shop and it made that ratcheting sound when we fired it up. He insists that the noise is coming from the cruise control assembly that sits under that plastic cover on the left side valve cover. And it seems most prevalent when backing off the throttle. I was looking through the F body service manual and it appears the device does have some sort of cabling on a type of spool, maybe something in there has come loose. Not sure why it would stop after the engine has been running for a few minutes.

Pulled the fuse when it was making the noise and it didn't make a difference, so at this point a problem with the air pump is moved down the list of possible causation.

Didn't have the missing problem crop up in the brief time we had to look at it. Probably will not get to do much until next week anyway with a holiday tomorrow.

 

The black box on the drivers valve cover is the traction control (option NW9) servo. The throttle cable from the accel pedal and the cable from the cruise control box (under car on drivers side front frame rail) . The traction control modulates the inputs from those two cables when traction control activates.

Try using the button to turn the traction control system off when you start the car.

 

I will give that a try tomorrow, thank you.

 

To update, put a fuel pressure gauge on the Schrader valve and it never gets above 30 PSI, it gets to 30 PSI and stays there. My mechanic is diagnosing it as a faulty fuel pump. Before I go ahead with the repair(which looks like a lot of work) I wanted to check back with the brain trust here though. He believes since the pressure is holding at 30 PSI the fuel pressure regulator is operating as it should and is probably fine. No fuel comes out of the vacuum line(to the FPR) either, another signal the FPR is operating correctly from what I understand.

If it is the fuel pump is the following Delphi part suitable,

More Information for DELPHI HP10033

or should I spend the extra money on the AC Delco part?

More Information for ACDELCO FLS1055

Also, given the work required to replace the in-tank pump, is an outboard "after market" pump an option, possibly a less expensive option?

Thanks

 

Under what conditions was the fuel pressure measured:

-prime pressure after turning the key to run, but not starting the engine

-at idle with the vacuum compensation line disconnected

-at idle with the vacuum compensation connected to the FPR

-on the road with the engine under load?

You can't put an inline pump between the existing in tank pump and the engine. The dead in tank pump will reduce flow to the inline pump, and fuel can flash to vapor in the line,

What ever happened to the "ratcheting noise"? What was it?

 

I wasn't at the shop when the FP test was done, out of town business, but back home and headed over there today. Not sure of the conditions, quick phone call, little detail.

Apparently the ratcheting sound has been diagnosed as an exhaust leak although I didn't ask why it goes away after warming up. Thinking about about I'm guessing once the metals/gaskets involved warm up the leak gets "sealed". Will also find out more about this later today.

I believe the FP test was done key on, engine off. Not sure of the vacuum line status when done.

Sorry for the lack of detail, I've been trying to manage this from afar and it's proving to be somewhat difficult, I hope to have some hard data today.

Researching the site I found this post that indicates 43.5 PSI vac line off is GM spec but I don't see a definitive rate for line on and/or at idle. Again, I have the GM factory F-body service manuals but they are at the aforementioned shop so I can't check the expected FP rates.

 

I thought of the exhaust leak. As metal parts heat up, they expand, closing up the gap between the manifold-gasket-head. You really need to get the gasket replaced and the bolts tightened up. An exhaust leak before the O2 sensors causes a "false lean" condition, and the PCM starts to add fuel that isn't really needed = runs rich.

Fuel pressure:

-turn key to run to prime pump, it will shut off after a couple seconds. Should reach greater than 40psi at shut off, and not lose pressure rapidly when the pump shuts off. Rapid blead down of pressure will cause hard starting, but typically it's due to a bad check valve in the fuel pump, which will not affect the fuel system once the engine starts. Could also be due to leaking injectors, faulty fuel pressure regulator, leaking lines, including the flex hose in the tank.

-at idle, vacuum line off, you want 43.5psi (3 bar). GM spec says anything between 41-47psi is OK. Hold finger over vacuum line to prevent a vacuum leak while testing.

-reattach vacuum line. Pressure will drop proportional to intake manifold vacuum. With a stock cam, and 8psi drop wouldn't be unreasonable. With a mote aggressive cam, the pressure will drop less.

-tape gauge to outside of windsheld, take it out on the road, wide open throttle above 5,000 RPM and pressure should not drop below 40psi.

When you are driving at part throttle, the fuel pressure will increase as you press on the throttle, and intake manifold drops. When you close throttle, and intake manifold vacuum increases, fuel pressure will drop. Point is, it's supposed to fluctuate with throttle position.

 

Turn key on for second or two then off. Waited about 10 secs turned key on then off, and then turned on again, with engine off.

Fuel pressure never got above 25 PSI, he said it was almost 30 PSI yesterday but seems to be getting progressively lower. The gauge was reading 30 PSI when I walked up to the car when arriving there today.
Started the engine, with the above fuel pressure of 25 PSI and it was missing badly. This was with vacuum line attached, never got as far as disconnecting the line. It seemed to drop to around 15 PSI when idling but never got above 25 PSI under any condition. Did not drive given the bad missing condition.

The pressure is holding at the above mentioned 25 PSI with key on and off so the assumption is the fuel pressure regulator is functioning as it should.

The mechanic is a good friend but he was busy and seems very positive it's the fuel pump so I didn't push for any further diagnostics. He owns a '95 Impala with the very same engine so he is somewhat familiar with the fuel system. Had to replace the fuel pump in his car as well, said the wires were fused and blackened from getting too hot.

 

An update on my above issues.

Replaced the fuel pump and drove the car home and it ran fine. Try to start it the next day and nothing, it just cranks and cranks. Tried all the tricks I've learned here and still nothing. Towed back to shop and apparently the opti-spark is bad, below are some images.







The unit had moisture inside as well as looking like the above. I've never seen the inside of one of these before but the above looks like a unit in very bad condition, please correct me if I'm wrong.

Purchased a Delphi CZ20003 (from Amazon no less, $256) brand new and plan to Loctite the rotor screws before install.

Also decided to replace the water pump and the timing cover seals. Purchased the water pump drive gear seal installer off of Ebay, couldn't find it anywhere else. Also installing new spark plugs(NGK Iridium) and wires.

A few questions,

Since all of the above has been removed I got to thinking about the timing chain. The car has 120K on it but ran very well before the starting issue started. Am I making a mistake by not replacing the timing gear set?

Also, when reading this post here, it mentions that the orientation of that water pump drive seal is to have both the front and rear lip facing towards the front of the motor, not the inner facing in and the outer facing forward. Is that correct? I thought I had read that the inner was to suppose to slope towards the motor, this \ instead of this / ?

Replacing the timing chain will add quite a bit to an already increasing bill, but if it is the wise play I'll do it. But if it's not really needed I'd like to avoid it.

Finally, the ratcheting sound earlier appears to be indeed an exhaust leak. I noticed that No. 8 exhaust had some blackening where the manifold meets the head and sure enough when putting a wrench on it it just spun in the hole. It appears to have snapped off just inside the manifold and not at the manifold/head mating point. My mechanic seems fairly confident he can do a repair without major surgery like removing the head etc.

So, looking for input on the condition of the Opti, does it look as if it was the no-start culprit, the timing chain question and is there an "easy" way to repair the exhaust manifold bolt situation?

Regarding the timing chain question, I don't race the car, it's driven on a semi-daily basis, I will have some fun "opening it up" on occasion, but never really drive it hard, at least not for any length of time.

As always, thanks for all the input!

 

To followup and finish this thread I just wanted to post that the car is running great now. Replaced the Opti-spark, new Iridium plugs, new wires, a new Mallory coil, new water pump and radiator. And fixed the ratcheting noise which was indeed an exhaust leak at the No.8 exhaust port.

The car runs great, in fact I hadn't really noticed how much performance I had been missing, no doubt that bad Opti was the major issue. It runs so good I've had renewed interest in driving, especially with summer coming!

Thanks all for your help!

Jim

 

Glad to hear you got it sorted out.