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Normal ideal, boggs above half throttle, can cover tbi and…

Normal ideal, boggs above...

Normal ideal, boggs above half throttle, can cover tbi and doesn't change idle

Mechanic's Assistant: How many miles are on the car? What size is the engine?

88k, 454

Mechanic's Assistant: What is the make/model/year of your car?

90 chevy p30 motorhome chassis

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

Yes, replaced intake gasket, rebuilt tbi, new plugs,wires,distributor cap and button, new ignition coil

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

This all started after a breakdown of ignition coil and backfiring through exhaust and intake

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Answered in 2 minutes by:
3/10/2018
AMEDEE
AMEDEE, CHEVY TECHNICIAN
Category: Chevy
Satisfied Customers: 28,142
Experience: ASE certified tech ADVANCED LEVEL SPECIALIST. Wisconsin certified emissions state inspector
Verified

Hello and welcome!

Thank you for your question. My name is ***** ***** I will be happy to assist you today. I am reviewing the details now.

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Have you checked the fuel pressure? You might just have a weak fuel pump..

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Customer reply replied 4 months ago
New pump and filter, 20 psi right past filter. When I first bought this I could kill engine by putting my hands over tbi. Now there is no suction felt. I put vac gauge on it and got 18lbs
Customer reply replied 4 months ago
The iac I can see is bottomed out closed. I can unhook it and nothing changes.

Thank you for that information.

What is the fuel pressure at going directly into the throttle body?

Have you checked for plugged exhaust? What is the back exhaust pressure at?

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Customer reply replied 4 months ago
There is no tap or way to check at tbi, I haven't checked back pressure on exhaust. Exhaust pours out as normal

Ok, you will need to do this to continue. Sounds to me like the fuel pressure is low, the exhaust is plugged or maybe even the injectors could be faulty.

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Customer reply replied 4 months ago
New injectors, total tbi rebuild, when you go above 1/2 throttle is makes an enormous suction sound. But I'm still confused by putting a bag over tbi the idle doesn't change. I think one deals with the other

Would you be willing to speak on the phone?

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Customer reply replied 4 months ago
Not really

Ok no problem. Please run these test and reply back with the results.

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Customer reply replied 4 months ago
K, hit me up

.

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Customer reply replied 4 months ago
What test do I need to do?
Customer reply replied 4 months ago
How do I do an exhaust back pressure test

Hook up a, exhaust back pressure tester into the oxygen sensor whole after removing the oxygen sensor, bring the engine rpm to 2,000 rpm and take a reading.

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Customer reply replied 4 months ago
K, let me see if I can find one

ok

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Customer reply replied 4 months ago
I replaced pump and filter after this started to no change

ok

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Customer reply replied 4 months ago
Since there is no cat on this do I still need to do exhaust test

I would still recommend it.

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Customer reply replied 4 months ago
K, I'll build one due to no one has one

Why dont you just order one?

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Customer reply replied 4 months ago
I've gotta fix today or tomorrow to live in for work out of town
Ok
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Customer reply replied 4 months ago
2 months ago I did 1300 mile trip. No problem but half throttle issue, lacking power, got 7 mpg as normal
Customer reply replied 4 months ago
Could it be 180 out?

What be 180 out?

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Customer reply replied 4 months ago
Distributor, it wouldn't run, checking fuel pressure now
Customer reply replied 4 months ago
12 lbs of fuel at idle
Customer reply replied 4 months ago
I'm still thinking there is a vaccum leak, due to covering tbi with a bag doesn't change idle any at all

ok

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Customer reply replied 4 months ago
next thought of checking
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,694
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
Verified

They do make fuel pressure testers that connect to the TBI, and I would install one, and road test it with the engine cover off so I could watch the gauge when the power drops. I want to see what the fuel pressure is doing when the demand is high. I understand you just put a pump and filter in it, but you still can't know that it's good until you test it.

These are easy systems to work on, the distributor and the fuel pressure are the key components here to make the whole thing work. Fuel pressure is far and away the most common issue, so we want to make sure we "know" what the fuel pressure is when it's acting up.

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Customer reply replied 4 months ago
Putting a bag covering the whole tbi doesn't starve the engine

The idle air enters through a tiny kidney shaped passage on the side, just above the IAC valve in the throttle body. At idle, there is nearly no air going past the throttle blades, it's going in through the IAC passage. It's not a valid test, if there was no air going into the engine, it simply wouldn't be running.

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Customer reply replied 4 months ago
I blocked the iac passage off and it idled down but didn't die. So I guess that's good.

Yes, that all sounds normal.

Can you get a TBI pressure gauge and put on it?

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Customer reply replied 4 months ago
I have a fuel pressure kit but it's made to go at filter. Between filter and tbi. I followed the line all the way to tbi and no filters. It's hard line from tbi to frame and has 2' of hose then back to metal to filter

Yes, I am familiar with the setup. The TBI gauge plumbs in right at the throttle body, it has special adapters made to do this, and you can probably borrow this at no charge from a parts store like Autozone or Advance.

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Customer reply replied 4 months ago
Let me check

ok

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Customer reply replied 4 months ago
None of them have it to do at tbi, I'll have to order an adapter to do this test. They all have same filter set up

It's important, I wouldn't belabor it if I didn't believe there's a fuel delivery problem here. Do you want to order it up and revisit this when you get the gauge you need?

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Customer reply replied 4 months ago
I need to install it and go down the road getting readings on it

Yes, all testing needs to take place while the vehicle is acting up. So, put the gauge on it, leave the engine cover off (for convenience) and drive it down the road. When it bogs, look at the gauge and see what the fuel pressure is doing.

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Customer reply replied 4 months ago
Ok, so I moved the coil and got it hooked up. I'm getting 10.8 to 11 lbs pressure at idle. I will do road test

OK!

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Customer reply replied 4 months ago
Pressure gauge never moved off 10.8 to 11. Maybe it's fine and I'm thinking more power but mh weighs 16k. Think I'll bump up to 8 degrees on timing
Customer reply replied 4 months ago
Maybe I've forgotten that it's gonna make noise when you really get butterflies open

Don't adjust timing on this! There is only one timing setting, you haven't been moving the distributor to try to fix this, have you?

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I have owned a 1990 Class A, with exactly the powertrain you're talking about. Yes, when the throttle plates open, it's going to roar. Normal.

It's also not going to be a powerhouse, but it shouldn't lose power as you open the throttle more.

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Customer reply replied 4 months ago
Timing is set to 4 btdc. Just seems sluggish in a hill. Get it up to speed at 60 and it roars right along

How long is the coach?

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Customer reply replied 4 months ago
33'
Customer reply replied 4 months ago
Tag axle

I see you have GVW of 16k. I think that's what mine was as well. I can tell you that I got passed on every hill by the diesel pusher guys, the BBC is good for what it is, but it's not a monster powerplant. I've actually owned two of them, one was 23' and one was 29'. The 23' did pretty good, but the 29' had the 16k GVW, and it struggled on hills. 33' is a lot of motorhome for a BBC.

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Customer reply replied 4 months ago
I guess we have a clean bill of health or should I test more

You can do the normal things to improve HP on this: a free flowing air filter setup, open up the exhaust. Consider a set of headers and 3" pipe all the way to the rear. But I think you do have a pretty clean bill of health, and it sounds like it's running as well as can be expected, considering what it's being asked to carry up hill.

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Customer reply replied 4 months ago
K, thanks

You're welcome.

Please take a moment to rate my answers by clicking the stars at the top of the page, that's how I get compensated. If you need anything else, let me know, I'm happy to follow up.

You will also have an opportunity to add a tip to the payment if you wish, and that is greatly appreciated. I work very hard to make sure you're getting the very best information I can provide for you.

Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,694
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
Verified
Jerry Newton and 87 other Chevy Specialists are ready to help you
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Customer reply replied 4 months ago
I was wrong in problem resolved, definitely not running correct. Took on 120 mile trip and above half throttle is horrible.

Do you have access to a scan tool of any kind that will connect to this rig? It would be a GM OBD 1 scan tool, I'd want to be able to look at some data.

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Customer reply replied 4 months ago
I don't know if autozone has one to do obd1

Give them a call, I bet they do. When this thing is acting up, I want to know what fuel trim (in those days, it was known as block learn) is doing. Is it going lean, or is it going rich? When it's acting up, if you give it a shot of fuel does it get better or worse, or no change at all?

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Customer reply replied 4 months ago
It loses power, as soon as you let off throttle some it gets going, but give it throttle immediately it just loses power. Like the air/fuel are out of wack

That's what I am wondering about, is it lean? Oxygen sensor doesn't do anything at wide open throttle, this thing should just be dumping fuel down the throttle body to power that engine. The only thing I'm unsure about is MAP sensor, I would want to make sure it doesn't have a split vacuum hose or anything like that, with the scan tool I'd want to make sure it's measuring barometric pressure accurately. The MAP sensor vacuum nipple also has a tendency to get clogged up inside the throttle body, and will restrict this important vacuum signal. You have to remove the throttle body and flip it over to clean that passage out, if you think that's an issue.

If you can get a scan tool, depending on where you live, barometric pressure should be about 30" of mercury as read on the tool, and at WOT, the MAP and BARO readings should be the same, because there is no vacuum in an engine at WOT.

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Customer reply replied 4 months ago
Ok, let me work on that and get back to you. Be wknd probably, working 12 hrs a day

That's fine, I'll follow up when I hear back from you.

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Customer reply replied 4 months ago
K, thanks
Customer reply replied 2 months ago
Finally got a obd1 scanner. Letting it idle it's surging between 950- 1050 rpms, iac is 32-34, map is 26.14, temp is 183. If I floor it the map drops to 26.11.
Customer reply replied 2 months ago
Delete last information, true data is 185 temp, open loop status, 875 rpm steady, 26.12 map inches, no change when rev up to 5k rpm
Customer reply replied 2 months ago
Rich/lean is rich

The MAP value never changes much from 26 inches? That's a problem.

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Customer reply replied 2 months ago
don't know if bad or another problem.

The MAP is bad, or the MAP vacuum supply hose is broken, plugged up, something. There's something going on there.

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Customer reply replied 2 months ago
I should be able to suck on tube? If I can then I'll change map sensor

You should have a strong vacuum supply at the vacuum hose that connects to the MAP sensor. With key on/engine off, the reading should be dramatically different from when it's running, and when you zing the throttle, it should also move substantially.

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Customer reply replied 2 months ago
No change with just key. It moves 1 inch of hg. When I zing it just might move 1"

Check the vacuum supply, and if it's good, put a MAP sensor in it.

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Customer reply replied 2 months ago
K, I'll let you know what happens
Customer reply replied 2 months ago
I was not driving it with those readings. I was just revving it in idle. Do i need to drive it to get accurate readings?
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